Device and method for preventing magnetic-resonance imaging induced damage

ABSTRACT

An electromagnetic shield has a first patterned or apertured layer having non-conductive materials and conductive material and a second patterned or apertured layer having non-conductive materials and conductive material. The conductive material may be a metal, a carbon composite, or a polymer composite. The non-conductive materials in the first patterned or apertured layer may be randomly located or located in a predetermined segmented pattern such that the non-conductive materials in the first patterned or apertured layer are located in a predetermined segmented pattern with respect to locations of the non-conductive materials in the second patterned or apertured layer.

CROSS-REFERENCE TO RELATED U.S. PATENT APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 10/405,154, filed on Apr. 2, 2003. The entire contents of U.S.patent application Ser. No. 10/405,154 are hereby incorporated byreference.

FIELD OF THE PRESENT INVENTION

The present invention relates generally to a device and method forpreventing magnetic-resonance imaging induced damage. More particularly,the present invention is directed to medical assist systems, which mayinclude leads and other implantable or non-implantable components, thatare shielded by segmented shielding to hardened or immune the systemsfrom electromagnetic interference or insult, namely electromagneticinterference or insult in a magnetic-resonance imaging environment andto a modifiable magnetic-resonance imaging, which is, automatically ormanually, responsive to sensed tissue temperature changes or knownlocalized specific energy absorption ratios.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

The subject matter of the present application is related to the subjectmatter of co-pending U.S. patent application Ser. No. 09/885,867, filedon Jun. 20, 2001, entitled “Controllable, Wearable magnetic-resonanceimaging-Compatible Cardiac Pacemaker With Pulse Carrying PhotonicCatheter And VOO Functionality”; co-pending U.S. patent application Ser.No. 09/885,868, filed on Jun. 20, 2001, entitled “Controllable, Wearablemagnetic-resonance imaging-Compatible Cardiac Pacemaker With PowerCarrying Photonic Catheter And VOO Functionality”; co-pending U.S.patent application Ser. No. 10/037,513, filed on Jan. 4, 2002, entitled“Optical Pulse Generator For Battery Powered Photonic Pacemakers AndOther Light Driven Medical Stimulation Equipment”; co-pending U.S.patent application Ser. No. 10/037,720, filed on Jan. 4, 2002, entitled“Opto-Electric Coupling Device For Photonic Pacemakers And OtherOpto-Electric Medical Stimulation Equipment”; co-pending U.S. patentapplication Ser. No. 09/943,216, filed on Aug. 30, 2001, entitled “PulseWidth Cardiac Pacing Apparatus”; co-pending U.S. patent application Ser.No. 09/964,095, filed on Sep. 26, 2001, entitled “Process for ConvertingLight”; co-pending U.S. patent application Ser. No. 09/921,066, filed onAug. 2, 2001, entitled “magnetic-resonance imaging-Resistant ImplantableDevice”; co-pending U.S. patent application Ser. No. 10/077,842, filedon Feb. 19, 2002, entitled “An Electromagnetic Interference ImmuneTissue Invasive System”; co-pending U.S. patent application Ser. No.10/077,823, filed on Feb. 19, 2002, entitled “An ElectromagneticInterference Immune Tissue Invasive System”; co-pending U.S. patentapplication Ser. No. 10/077,887, filed on Feb. 19, 2002, entitled “AnElectromagnetic Interference Immune Tissue Invasive System”; co-pendingU.S. patent application Ser. No. 10/077,883, filed on Feb. 19, 2002,entitled “An Electromagnetic Interference Immune Tissue InvasiveSystem”; and co-pending U.S. patent application Ser. No. 10/077,958,filed on Feb. 19, 2002, entitled “An Electromagnetic Interference ImmuneTissue Invasive System”.

The entire content of each of the above noted co-pending U.S. patentapplications (Ser. Nos. 09/885,867; 09/885,868; 10/037,513; 10/037,720;09/943,216; 09/964,095; 09/921,066; 10/077,842; 10/077,823; 10/077,887;10/077,883; and 10/077,958) is hereby incorporated by reference.

BACKGROUND OF THE PRESENT INVENTION

Magnetic-resonance imaging (“magnetic-resonance imaging”) has beendeveloped as an imaging technique adapted to obtain both images ofanatomical features of human patients as well as some aspects of thefunctional activities of biological tissue. These images have medicaldiagnostic value in determining the state of the health of the tissueexamined.

In a magnetic-resonance imaging process, a patient is typically alignedto place the portion of the patient's anatomy to be examined in theimaging volume of the magnetic-resonance imaging apparatus. Such anmagnetic-resonance imaging apparatus typically comprises a primarymagnet for supplying a constant magnetic field (B₀) which, byconvention, is along the z-axis and is substantially homogeneous overthe imaging volume and secondary magnets that can provide linearmagnetic field gradients along each of three principal Cartesian axes inspace (generally x, y, and z, or x₁, x₂ and X₃, respectively). Theapparatus also comprises one or more radio-frequency coils that provideexcitation and detection of the magnetic-resonance imaging signal.

The use of the magnetic-resonance imaging process with patients who haveimplanted or non-implanted medical assist devices; such as, but notlimited to, cardiac assist devices, implanted insulin pumps, catheterguide wires, leads for neurostimulation probes, intraluminal coils,guided catheters, temporary cardiac pacemakers, temporary esophagealpacemakers; often presents problems.

For a specific example, as is known to those skilled in the art,implantable devices (such as implantable pulse generators (IPGs) andcardioverter/defibrillator/pacemakers (CDPs)) are sensitive to a varietyof forms of electromagnetic interference (EMI) because these enumerateddevices include sensing and logic systems that respond to low-levelelectrical signals emanating from the monitored tissue region of thepatient and that these devices may also have metal wire leads, which canact as antenna and provide a path for the induced energy to travel toand possibly damage power sensitive circuitry.

Since the sensing systems and conductive elements of these medicalassist devices are responsive to changes in local electromagneticfields, the medical assist devices are vulnerable to external sources ofsevere electromagnetic noise, and in particular, to electromagneticfields emitted during the magnetic-resonance imaging (magnetic-resonanceimaging) procedure. Thus, patients with medical assist devices aregenerally advised not to undergo magnetic-resonance imaging(magnetic-resonance imaging) procedures.

To more appreciate the problem using a specific illustration, namely theuse of implantable cardiac assist devices during a magnetic-resonanceimaging process, will be briefly discussed.

The human heart may suffer from two classes of rhythmic disorders orarrhythmias: bradycardia and tachyarrhythmia. Bradycardia occurs whenthe heart beats too slowly, and may be treated by a common implantablepacemaker delivering low voltage (about 3 V) pacing pulses.

The common implantable pacemaker is usually contained within ahermetically sealed enclosure, in order to protect the operationalcomponents of the device from the aqueous environment of the body, aswell as to protect the body from the device.

The common implantable pacemaker operates in conjunction with one ormore electrically conductive leads, adapted to conduct electricalstimulating pulses to sites within the patient's heart and tocommunicate sensed signals from those sites back to the implanteddevice.

Furthermore, the common implantable pacemaker typically has a metal caseand a connector block mounted to the metal case that includesreceptacles for leads which may be used for electrical stimulation orwhich may be used for sensing of physiological signals. The battery andthe circuitry associated with the common implantable pacemaker arehermetically sealed within the case. Electrical interfaces are employedto connect the leads outside the metal case with the medical devicecircuitry and the battery inside the metal case.

Electrical interfaces serve the purpose of providing an electricalcircuit path extending from the interior of a hermetically sealed metalcase to an external point outside the case while maintaining thehermetic seal of the case. A conductive path is provided through theinterface by a conductive pin that is electrically insulated from thecase itself.

Such interfaces typically include a ferrule that permits attachment ofthe interface to the case, the conductive pin, and a hermetic glass orceramic seal that supports the pin within the ferrule and isolates thepin from the metal case.

A common implantable pacemaker can, under some circumstances, besusceptible to electrical interference such that the desiredfunctionality of the pacemaker is impaired. For example, commonimplantable pacemaker requires protection against electricalinterference from electromagnetic interference (EMI), defibrillationpulses, electrostatic discharge, or other generally large voltages orcurrents generated by other devices external to the medical device. Asnoted above, more recently, it has become crucial that cardiac assistsystems be protected from magnetic-resonance imaging sources.

Such electrical interference can damage the circuitry of the cardiacassist systems or cause interference in the proper operation orfunctionality of the cardiac assist systems. For example, damage mayoccur due to high voltages or excessive currents introduced into thecardiac assist system by voltages or currents induced in the cardiacassist system circuitry or on the wire leads leading to and from thecardiac assist system circuitry.

Therefore, it is required that such voltages and currents be limited atthe input of such cardiac assist systems, e.g., at the interface,Protection from such voltages and currents has typically been providedat the input of a cardiac assist system by the use of one or more zenerdiodes and one or more filter capacitors.

For example, one or more zener diodes may be connected between thecircuitry to be protected, e.g., pacemaker circuitry, and the metal caseof the medical device in a manner which grounds voltage surges andcurrent surges through the diode(s). Such zener diodes and capacitorsused for such applications may be in the form of discrete componentsmounted relative to circuitry at the input of a connector block wherevarious leads are connected to the implantable medical device, e.g., atthe interfaces for such leads.

However, such protection, provided by zener diodes and capacitors placedat the input of the medical device, increases the congestion of themedical device circuits, at least one zener diode and one capacitor perinput/output connection or interface. This is contrary to the desire forincreased miniaturization of implantable medical devices.

Further, when such protection is provided, interconnect wire length forconnecting such protection circuitry and pins of the interfaces to themedical device circuitry that performs desired functions for the medicaldevice tends to be undesirably long. The excessive wire length may leadto signal loss and undesirable inductive effects. The wire length canalso act as an antenna that conducts undesirable electrical interferencesignals to sensitive CMOS circuits within the medical device to beprotected.

Additionally, the radio frequency (radio-frequency) energy that isinductively coupled into the wire causes intense heating along thelength of the wire, and at the electrodes that are attached to the heartwall. This heating may be sufficient to ablate the interior surface ofthe blood vessel through which the wire lead is placed, and may besufficient to cause scarring at the point where the electrodes contactthe heart. A further result of this ablation and scarring is that thesensitive node that the electrode is intended to pace with low voltagesignals becomes desensitized, so that pacing the patient's heart becomesless reliable, and in some cases fails altogether. Additionally, theswitching of the gradient magnetic fields may also induce unwantedvoltages causing problems with the circuitry and potential pacing of theheart.

Another conventional solution for protecting the implantable medicaldevice from electromagnetic interference is illustrated in FIG. 1. FIG.1 is a schematic view of an implantable medical device 12 embodyingprotection against electrical interference. At least one lead 14 isconnected to the implantable medical device 12 in connector block region13 using an interface.

In the case where implantable medical device 12 is a pacemaker implantedin a body 10, the pacemaker 12 includes at least one or both of pacingand sensing leads represented generally as leads 14 to sense electricalsignals attendant to the depolarization and repolarization of the heart16, and to provide pacing pulses for causing depolarization of cardiactissue in the vicinity of the distal ends thereof.

FIG. 2 more particularly illustrates the circuit that is usedconventionally to protect from electromagnetic interference. As shown inFIG. 2, protection circuitry 15 is provided using a diode arraycomponent 30. The diode array consists of five zener diode triggeredsemiconductor controlled rectifiers (SCRs) with anti-parallel diodesarranged in an array with one common connection. This allows for a smallfootprint despite the large currents that may be carried through thedevice during defibrillation, e.g., 10 amps. The SCRs 20-24 turn on andlimit the voltage across the device when excessive voltage and currentsurges occur.

Some of the zener diode triggered SCRs may be connected to anelectrically conductive pin, with each electrically conductive pin beingconnected to a medical device contact region to be wire bonded to padsof a printed circuit board. The diode array component 30 of FIG. 2 maybe connected to the electrically conductive pins via die contact regionsalong with other electrical conductive traces of the printed circuitboard.

Other attempts have been made to protect medical assist devices frommagnetic-resonance imaging fields. For example, U.S. Pat. No. 5,968,083(to Ciciarelli et al.) describes a device adapted to switch between lowand high impedance modes of operation in response to EMI insult.Furthermore, U.S. Pat. No. 6,188,926 (to Vock) discloses a control unitfor adjusting a cardiac pacing rate of a pacing unit to an interferencebackup rate when heart activity cannot be sensed due to EMI.

Another problem associated with magnetic-resonance imaging is thetemperature change in tissue regions caused by using conventionalmagnetic-resonance imaging techniques. When a substance such as humantissue is subjected to a static magnetic field, the individual magneticmoments of the spins in the tissue align in a parallel and anti-paralleldirection with the static magnetic field. This direction along thestatic magnetic field can be termed as the longitudinal direction. Inmagnetic-resonance imaging, the radio frequency polarizing field usedfor spin manipulation is constantly changing and thus, the individualmagnetic moments of the spins in the tissue attempt to align with thepolarizing field. The constant changing of alignment of the magneticmoments of the spins in the tissue causes the tissue's temperature toincrease, thereby exposing the tissue to possible magnetic-resonanceimaging induced thermal damage.

Although, conventional medical assist devices provide some means forprotection against electromagnetic interference, these conventionalmedical assist devices require much circuitry and fail to providefail-safe protection against radiation produced by magnetic-resonanceimaging procedures. Moreover, the conventional medical assist devicesfail to address the possible damage that can be done at the tissueinterface due to radio-frequency-induced heating. Furthermore, theconventional medical assist devices fail to address the unwanted tissueregion stimulation that may result from radio-frequency-inducedelectrical currents. Lastly, conventional magnetic-resonance imagingprocesses fail to provide a proper safeguard against potentialmagnetic-resonance imaging induced thermal damage due to the tissue'sexposure to the switching magnetic field gradients and the circularlypolarized Radio Frequency Field of the magnetic-resonance imagingprocess.

Thus, it is desirable to provide protection against electromagneticinterference, without requiring much circuitry and to provide fail-safeprotection against radiation produced by magnetic-resonance imagingprocedures. Moreover, it is desirable to provide medical assist devicesthat prevent the possible tissue damage that can be done at the tissueinterface due to induced electrical signals. Furthermore, it isdesirable to provide an effective means for transferring energy from onepoint in the body to another point without having the energy causing adetrimental effect upon the body. Lastly, it is desirable to implement amagnetic-resonance imaging process, which can be modified, automaticallyor manually, in response to sensed tissue temperature changes or knownlocalized specific energy absorption rates, so as to prevent possiblemagnetic-resonance imaging induced thermal damage.

SUMMARY OF THE PRESENT INVENTION

A first aspect of the present invention is directed to a medical assistsystem. The medical assist system includes a primary device housing, theprimary device housing having a control circuit therein; a lead systemto provide an electrical path between a tissue region and the primarydevice housing; and a shielding formed around the lead system to shieldthe lead system from electromagnetic interference. The shielding ispatterned with non-conductive materials and conductive material.

A second aspect of the present invention is directed to a medical assistdevice. The medical assist device includes a primary device housing, theprimary device housing having a control circuit therein, and a shieldingformed around the primary device housing to shield the primary devicehousing and any circuits therein from electromagnetic interference. Theshielding is patterned with non-conductive materials and conductivematerial.

A third aspect of the present invention is directed to anelectromagnetic shielded implantable lead. The electromagnetic shieldedimplantable lead includes an electrical lead and a shielding formedaround the electrical lead to shield the electrical lead fromelectromagnetic interference. The shielding is patterned withnon-conductive materials and conductive material.

A fourth aspect of the present invention is directed to anelectromagnetic shield. The electromagnetic shield includes a firstpatterned layer having non-conductive materials and conductive materialand a second patterned layer having non-conductive materials andconductive material.

A fifth aspect of the present invention is a magnetic-resonance imagingprocess. The magnetic-resonance imaging process images a tissue regionaccording to an image acquisition sequence and adjusts the imageacquisition sequence, in response to a predetermined parameter, to allowfor cooling of the imaged tissue region.

A sixth aspect of the present invention is a magnetic-resonance imagingprocess. The magnetic-resonance imaging process images a tissue regionaccording to an image acquisition sequence; adjusts the imageacquisition sequence, in response to a predetermined parameter, to allowfor cooling of the imaged tissue region; and shields components of amedical assist device with a shield patterned with non-conductivematerials and conductive material.

A seventh aspect of the present invention is a device for amplifying anelectrical signal of physiological significance in a magnetic-resonanceimaging environment. The device includes at least two electrodes withassociated input leads for coupling to a patient; an amplifier having azero-signal reference terminal for detecting and amplifying the desiredphysiological signal; and a filter, connected to the input leads andcoupling the at least two electrodes to the amplifier, to attenuate anyinduced radio-frequency signal produced in the magnetic-resonanceimaging environment and passing the lower frequency desired electricalphysiological signal. The filter includes a shield enclosing the filter.The shield enclosing the filter is patterned with non-conductivematerials and conductive material.

An eighth aspect of the present invention is a medical assist system.The medical assist device includes a primary device housing; the primarydevice housing having a control circuit therein; a lead system toprovide an electrical path between a tissue region and the primarydevice housing; a shielding formed around the lead system to shield thelead system from electromagnetic interference; and a filter, connectedto the lead system, to attenuate any induced radio-frequency signal andpassing a desired electrical physiological signal. The filter includes ashield enclosing the filter and a low pass filter connected to the leadsystem. The shielding is patterned with non-conductive materials andconductive material.

A ninth aspect of the present invention is a medical assist system. Themedical assist system includes a primary device housing, the primarydevice housing having a control circuit therein; a lead system toprovide an electrical path between a tissue region and the primarydevice housing; and a shielding formed around the lead system to shieldthe lead system from electromagnetic interference. The shielding is anapertured conductive material having a maximum aperture dimension of0.01 millimeters to 10 millimeters.

A tenth aspect of the present invention is a medical assist device. Themedical assist device includes a primary device housing, the primarydevice housing having a control circuit therein; and a shielding formedaround the primary device housing to shield the primary device housingand any circuits therein from electromagnetic interference. Theshielding is an apertured conductive material having a maximum aperturedimension of 0.01 millimeters to 10 millimeters.

An eleventh aspect of the present invention is an electromagneticshielded lead. The electromagnetic shielded lead includes an electricallead and a shielding formed around the electrical lead to shield theelectrical lead from electromagnetic interference. The shielding is anapertured conductive material having a maximum aperture dimension of0.01 millimeters to 10 millimeters.

A twelfth aspect of the present invention is an electromagnetic shield.The electromagnetic shield includes a first apertured conductivematerial having a maximum aperture dimension of 0.01 millimeters to 10millimeters and a second apertured conductive material having a maximumaperture dimension of 0.01 millimeters to 10 millimeters.

A further aspect of the present invention is a magnetic-resonanceimaging process. The magnetic-resonance imaging process images a tissueregion according to an image acquisition sequence; adjusts the imageacquisition sequence, in response to a predetermined parameter, to allowfor cooling of the imaged tissue region; and shields components of amedical assist device with a shield being an apertured conductivematerial and having a maximum aperture dimension of 0.01 millimeters to10 millimeters.

A still further aspect of the present invention is a device foramplifying an electrical signal of physiological significance in amagnetic-resonance imaging environment. The device includes at least twoelectrodes with associated input leads for coupling to a patient; anamplifier having a zero-signal reference terminal for detecting andamplifying the desired physiological signal; and a filter, connected tothe input leads and coupling the at least two electrodes to theamplifier, to attenuate any induced RF signal produced in themagnetic-resonance imaging environment and passing the lower frequencydesired electrical physiological signal. The filter includes a shieldenclosing the filter. The shield is an apertured conductive materialhaving a maximum aperture dimension of 0.01 millimeters to 10millimeters.

Another aspect of the present invention is a medical assist system. Themedical assist system includes a primary device housing, the primarydevice housing having a control circuit therein; a lead system toprovide an electrical path between a tissue region and the primarydevice housing; a shielding formed around the lead system to shield thelead system from electromagnetic interference; and a filter, connectedto the lead system, to attenuate any induced RF signal and passing adesired electrical physiological signal. The filter includes a shieldenclosing the filter and a low pass filter connected to the lead system.The shielding is an apertured conductive material having a maximumaperture dimension of 0.01 millimeters to 10 millimeters.

BRIEFS DESCRIPTION OF THE DRAWINGS

The present invention may take form in various components andarrangements of components, and in various steps and arrangements ofsteps. The drawings are only for purposes of illustrating a preferredembodiment and are not to be construed as limiting the presentinvention, wherein:

FIGS. 1 and 2 are illustrations of conventional techniques used toprotect against electromagnetic interference;

FIG. 3 is a cross-sectional view of one embodiment of segmentedshielding of a wire lead according to the concepts of the presentinvention;

FIG. 4 is a cross-sectional view of another embodiment of segmentedshielding of a wire lead according to the concepts of the presentinvention;

FIG. 5 is a top view of a layer of an embodiment of the shielding,having a predetermined segmented pattern of conductive materials, for awire lead shielding according to the concepts of the present invention;

FIG. 6 is a top view of a layer of another embodiment of the shielding,having a predetermined segmented pattern of conductive materials, for awire lead according to the concepts of the present invention;

FIG. 7 is a top view of a layer of another embodiment of the shielding,having a random pattern of conductive materials, for a wire leadaccording to the concepts of the present invention;

FIG. 8 is a flowchart illustrating the altering of a magnetic-resonanceimaging acquisition process in response to a measured temperature of thetissue region being imaged, according to the concepts of the presentinvention;

FIG. 9 is a flowchart illustrating the altering of a magnetic-resonanceimaging acquisition process in response to a measured localized specificabsorption ratio of the tissue region being imaged, according to theconcepts of the present invention;

FIG. 10 is a block diagram of an ECG amplifier capable of operating inthe high static magnetic field, radio-frequency field, and gradientfield environment produced in a magnetic-resonance imaging system,according to the concepts of the present invention;

FIG. 11 is a block diagram of a pacemaker capable of operating in thehigh static magnetic fields, radio-frequency field, and gradient fieldenvironment produced in a magnetic-resonance imaging system, accordingto the concepts of the present invention;

FIG. 12 is a block diagram of an implantable pacemaker, designed forexternal programming, which operates safely in the environment producedin a magnetic-resonance imaging system, according to the concepts of thepresent invention;

FIGS. 13 through 18 are schematic drawings of various ECG lead and/orharness configurations designed for operation in the environmentproduced by an magnetic-resonance imaging system, according to theconcepts of the present invention;

FIGS. 19 and 20 are block diagrams of ECG amplifiers with a one-stageradio-frequency filter design, according to the concepts of the presentinvention, for use with dual unshielded leads;

FIGS. 21 and 22 are block diagrams of ECG amplifiers with a one-stageradio-frequency filter design, according to the concepts of the presentinvention, for use with dual shielded leads;

FIGS. 23 and 24 are block diagrams of ECG amplifiers with a multistageradio-frequency filter design, according to the concepts of the presentinvention, for use with dual unshielded leads;

FIGS. 25 and 26 are block diagrams of ECG amplifiers with a multistageradio-frequency filter design, according to the concepts of the presentinvention, for use with dual shielded leads;

FIGS. 27 and 28 are block diagrams of ECG amplifiers with a multistageradio-frequency filter design, according to the concepts of the presentinvention, for use with a multi-lead ECG harness;

FIG. 29 is a block diagram of a magnetic-resonance imaging safeimplantable pacemaker in accordance with the present invention;

FIG. 30 is a block diagram of a magnetic-resonance imaging safeimplantable pacemaker using a multistage radio-frequency filter,according to the concepts of the present invention;

FIG. 31 is a block diagram of an implantable stimulator capable ofoperating in a magnetic-resonance imaging system, in accordance with thepresent invention;

FIG. 32 is a cross-sectional view of one embodiment of an aperturedshielding of a wire lead according to the concepts of the presentinvention;

FIG. 33 is a cross-sectional view of another embodiment of an aperturedshielding of a wire lead according to the concepts of the presentinvention;

FIG. 34 is a top view of a layer of an embodiment of the aperturedshielding, having a predetermined apertured pattern of non-conductivematerials, for a wire lead shielding according to the concepts of thepresent invention;

FIG. 35 is a top view of a layer of another embodiment of the aperturedshielding, having a predetermined apertured pattern of non-conductivematerials, for a wire lead according to the concepts of the presentinvention; and

FIG. 36 is a top view of a layer of another embodiment of the aperturedshielding, having a random apertured pattern of non-conductivematerials, for a wire lead according to the concepts of the presentinvention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

As noted above, the present invention is directed to an implantabledevice, such as a medical assist device, that is immune or hardened toelectromagnetic insult or interference.

For a general understanding of the present invention, reference is madeto the drawings. In the drawings, like reference have been usedthroughout to designate identical elements. In describing the presentinvention, the following term(s) have been used in the description.

For the purposes of the description below and the appended claims, theterm, medical assist device/system or tissue invasive device/system,refers to any device/system that may enable monitoring of livingtissue(s) or living, system(s) wherein the monitoring may be, but notlimited to an EKG signal, an ECG signal, a glucose level, hormone level,cholesterol level, or magnetic resonance image. The medical assistdevice/system or tissue invasive device/system may also enable stimulusintervention to provide assistance to living tissue(s) or livingsystem(s) so that the stimulus causes the selected body tissue or systemto function as desired. The stimulus may be, but not limited to, acardiac stimulating substance or electrical pulse, a blood thinningsubstance, insulin, estrogen, progesterone, or testosterone.

The medical assist device/system or tissue invasive device/system mayalso provide therapeutic treatment to the living tissue(s) or livingsystem(s) so that the treatment causes the selected body tissue orsystem to function as desired. The stimulus may be, but not limited to,radio frequency or laser ablation.

Furthermore, the medical assist device/system or tissue invasivedevice/system may be implanted in a body cavity of a living organism,either temporarily or permanently, or subcutaneously implanted into aliving organism either temporarily or permanently. Moreover, the medicalassist device/system or tissue invasive device/system may be locatedexternal to the living organism. Examples of medical assistdevices/systems or tissue invasive devices/systems are, but not limitedto, wearable or implantable cardiac pacers (such as pacemakers),implantable pulse generators (IPGs),cardioverter/defibrillator/pacemakers (CDPs), cardiac monitoringsystems, insulin pump controllers, brain monitoring systems, cardiacassist devices, implanted insulin pumps, catheter guide wires, leads forneurostimulation probes, intraluminal coils, guided catheters, temporarycardiac pacemakers, temporary esophageal pacemakers, etc.

Although the thrust of the description of one embodiment of the presentinvention, below, will use a cardiac assist system as an example of theworkings of the concepts of the present invention, the concepts aredirected to any medical assist device/system, tissue invasivedevice/system or tissue interactive device/system which includeselectrical components that require shielding from electromagneticinterference or insult such that the interference or insult could causedamage to the electrical components or damage to the surrounding tissue.

As noted above, a typical medical device may be a pacemaker. Thepacemaker may include at least one or both of pacing and sensing leadsrepresented generally as leads to sense electrical signals attendant tothe depolarization and repolarization of the heart and to provide pacingpulses for causing depolarization of cardiac tissue in the vicinity ofthe distal ends thereof.

The pacemaker or cardiac assist device may include a primary devicehousing. The primary device housing may include a control circuit, suchas a microprocessor integrated circuit for controlling the operations ofthe cardiac assist system. The control circuit may select a mode ofoperation for the cardiac assist system based on predetermined sensedparameters. The primary device housing may also include circuitry todetect and isolate cross talk between device pulsing operations anddevice sensing operations. The control circuit may isolate physiologicalsignals using a noise filtering circuit or a digital noise filtering.

The control circuit can be programmable from a source external of theprimary device housing or the control circuit can provide physiologicaldiagnostics to a source external of the primary device housing.

The primary device housing may include a power source. The power sourcemay be a battery power source in combination with a battery power sourcemeasuring circuit. Further, the control circuit may automatically adjusta value for determining an elective replacement indication condition ofa battery power source such that the value is automatically adjusted bythe control circuit in response to a measured level of a state of thebattery power source, the measured level generated by the battery powersource measuring circuit connected to the battery power source.

As noted above, both the primary housing and the leads of the medicalassist device/system require protection or shielding fromelectromagnetic interference or insult. The present invention provides ashielding that substantially prevents electromagnetic interference fromdamaging any, contained therein, electrical components or damage to thetissue surrounding the medical assist device/system's primary housing orleads. The shielding is a compilation of patterned layers havingnon-conductive materials and conductive materials contained in eachpatterned layer.

As is known is well known, conductive material provides a shield orblock for electromagnetic radiation, especially radio frequencyradiation (“radio-frequency”) or magnetic radiation (“magnetic-resonanceimaging”) used in magnetic-resonance imaging so that themagnetic-resonance imaging radiation cannot penetrate the conductivematerials to do damage to any electrical components located on the otherside of the shield.

However, although conductive material may provide a block to themagnetic-resonance imaging radiation so as to prevent penetration of themagnetic-resonance imaging radiation, the blocking of themagnetic-resonance imaging radiation can cause eddy currents to beinduced in the conductive material by the changing magnetic fields inthe magnetic-resonance imaging radiation. The eddy currents may causeheating of the conductive material, thereby heating the enclosedelectrical components so as to cause damage or heating the surroundingtissue so as to cause tissue damage.

To prevent the eddy currents from being induced to a level, which maycause damage from excessive heat, the conductive material in theshielding of the present invention is patterned, apertured, orsegmented. More specifically, magnetic-resonance imaging radiationshields generally contain thin layers of conductive cladding sometimesseparated by insulators or dielectrics. In the present invention, theconductive material in a layer is patterned, apertured, or segmented sothat the conductive material is literally broken up to limit the buildup of eddy currents induced by the changing magnetic fields producedduring magnetic-resonance imaging.

An example of such a patterned or segmented shielding for use with anelectrical lead is illustrated in FIG. 3. As shown in FIG. 3, anelectrical lead 60 is initially coated with an insulation layer 62 so asto electrically insulate the electrical lead from its surroundings. Uponthe insulation layer 62, a first patterned or segmented layer 80 ofshielding is placed or formed thereon. The first patterned or segmentedlayer 80 includes conductive materials 64 and non-conductive material(s)66.

Upon the first patterned or segmented layer 80 of shielding, a secondpatterned or segmented layer 82 of shielding is placed or formedthereon. The second patterned or segmented layer 82 includes conductivematerials 64 and non-conductive material(s) 66.

It is noted that the non-conductive material 66 may be formed of asingle integral piece of non-conductive material or be formed from amultitude of pieces of non-conductive material, the multitude of piecesbeing connected together in such a manner to function as a singleintegral piece of non-conductive material.

Upon the second patterned or segmented layer 82 of shielding, a thirdpatterned or segmented layer 84 of shielding is placed or formedthereon. The third patterned or segmented layer 84 includes conductivematerials 64 and non-conductive material(s) 66.

The conductive materials 64 of this embodiment of the present inventionmay be a metal, a carbon composite, nanotubes (wherein the nanotubes maybe constructed from a carbon base or the nanotubes could be formed fromother amalgams coated with the appropriate material(s)), metal-coatedcarbon filaments (wherein the metal may be one of the following metals:nickel, copper, cobalt, silver, gold, tin, or zinc), metal-coatedceramic filaments (wherein the metal may be one of the following metals:nickel, copper, cobalt, silver, gold, tin, or zinc), a composite ofmetal-coated carbon filaments and a polymer (wherein the polymer may beone of the following: polyether sulfone, silicone, polymide,polyvinylidene fluoride, or epoxy), a composite of metal-coated ceramicfilaments and a polymer (wherein the polymer may be one of thefollowing: polyether sulfone, silicone, polymide, polyvinylidenefluoride, or epoxy), a composite of metal-coated carbon filaments and aceramic (wherein the ceramic may be one of the following: cement,silicates, phosphates, silicon carbide, silicon nitride, aluminumnitride, or titanium diboride), a composite of metal-coated ceramicfilaments and a ceramic (wherein the ceramic may be one of thefollowing: cement, silicates, phosphates, silicon carbide, siliconnitride, aluminum nitride, or titanium diboride), or a composite ofmetal-coated (carbon or ceramic) filaments (wherein the metal may be oneof the following metals: nickel, copper, cobalt, silver, gold, tin, orzinc) and a polymer/ceramic combination (wherein the polymer may be oneof the following: polyether sulfone, silicone, polymide, polyvinylidenefluoride, or epoxy and the ceramic may be one of the following: cement,silicates, phosphates, silicon carbide, silicon nitride, aluminumnitride, or titanium diboride).

A more detailed description of the coated filaments is found in U.S.Pat. No. 5,827,997, entitled “Metal Filaments for ElectromagneticInterference Shielding.” The entire content of U.S. Pat. No. 5,827,997is hereby incorporated by reference.

The non-conductive material(s) 66 of this embodiment of the presentinvention may be a ceramic; glass; mica; anodized copper; metallicoxides; natural or synthetic rubbers; or resins, such as natural resins,epoxy resins, or silicones.

Over the shielding, a biocompatible layer 68 may be placed or formedthereon. Preferably, the biocompatible layer is a non-permeablediffusion resistant biocompatible material.

As illustrated in FIG. 3, the conductive materials 64 are segmented orpatterned in an x-direction, wherein the x-direction is a directionsubstantially parallel to an axis of the lead 60. In other words, theconductive materials 64 are broken up in this direction such that oneconductive material 64 is physically separated from a neighboringconductive material 64 in the same layer.

Moreover, as illustrated in FIG. 3, the conductive materials 64 ofdifferent layers are segmented or patterned in a y-direction, whereinthe y-direction is a direction substantially perpendicular to an axis ofthe lead 60. In other words, the conductive materials 64 of differentlayers are broken up in this direction such that one conductive material64 is electrically isolated from a neighboring conductive material 64 inan immediate adjacent layer; i.e., a conductive material 64 in the firstpatterned or segmented layer 80 is electrically isolated from aneighboring conductive material 64 in the second patterned or segmentedlayer 82.

Preferably, the x-directional gap between non co-layer immediatelyx-directional adjacent conductive materials 64 is much smaller than thewavelength of the electromagnetic pulse that the shield must attenuateto prevent the incident pulse from passing unattenuated through theshield.

Lastly, the conductive materials 64 may be segmented or patterned in az-direction, wherein the z-direction represents the planar surface of alayer as it is wrapped around the lead 60.

Another example of such a patterned or segmented shielding for use withan electrical lead is illustrated in FIG. 4. As shown in FIG. 4, anelectrical lead 60 is initially coated with an insulation layer 62 so asto electrically insulate the electrical lead from its surroundings. Uponthe insulation layer 62, a first patterned or segmented layer 80 ofshielding is placed or formed thereon. The first patterned or segmentedlayer 80 includes conductive materials 64 and non-conductive material(s)66.

As noted above, the non-conductive material 66 may be formed of a singleintegral piece of non-conductive material or be formed from a multitudeof pieces of non-conductive material, the multitude of pieces beingconnected together in such a manner to function as a single integralpiece of non-conductive material.

Upon the first patterned or segmented layer 80 of shielding, a layer 70of non-conductive material is placed or formed thereon. Upon the layer70 of non-conductive material, a second patterned or segmented layer 82of shielding is placed or formed thereon. The second patterned orsegmented layer 82 includes conductive materials 64 and non-conductivematerial(s) 66.

The conductive materials 64 of this embodiment of the present inventionmay be a metal, a carbon composite, nanotubes (wherein the nanotubes maybe constructed from a carbon base or the nanotubes could be formed fromother amalgams coated with the appropriate material(s)), metal-coatedcarbon filaments (wherein the metal may be one of the following metals:nickel, copper, cobalt, silver, gold, tin, or zinc), metal-coatedceramic filaments (wherein the metal may be one of the following metals:nickel, copper, cobalt, silver, gold, tin, or zinc), a composite ofmetal-coated carbon filaments and a polymer (wherein the polymer may beone of the following: polyether sulfone, silicone, polymide,polyvinylidene fluoride, or epoxy), a composite of metal-coated ceramicfilaments and a polymer (wherein the polymer may be one of thefollowing: polyether sulfone, silicone, polymide, polyvinylidenefluoride, or epoxy), a composite of metal-coated carbon filaments and aceramic (wherein the ceramic may be one of the following: cement,silicates, phosphates, silicon carbide, silicon nitride, aluminumnitride, or titanium diboride), a composite of metal-coated ceramicfilaments and a ceramic (wherein the ceramic may be one of thefollowing: cement, silicates, phosphates, silicon carbide, siliconnitride, aluminum nitride, or titanium diboride), or a composite ofmetal-coated (carbon or ceramic) filaments (wherein the metal may be oneof the following metals: nickel, copper, cobalt, silver, gold, tin, orzinc) and a polymer/ceramic combination (wherein the polymer may be oneof the following: polyether sulfone, silicone, polymide, polyvinylidenefluoride, or epoxy and the ceramic may be one of the following: cement,silicates, phosphates, silicon carbide, silicon nitride, aluminumnitride, or titanium diboride).

Over the shielding, a biocompatible layer 68 may be placed or formedthereon. Preferably, the biocdmpatible layer is a non-permeablediffusion resistant biocompatible material.

The non-conductive materials 66 of this embodiment of the presentinvention may be a ceramic; glass; mica; anodized copper; metallicoxides; natural or synthetic rubbers; or resins, such as natural resins,epoxy resins, or silicones.

As illustrated in FIG. 4, the conductive materials 64 are segmented orpatterned in an x-direction, wherein the x-direction is a directionsubstantially parallel to an axis of the lead 60. In other words, theconductive materials 64 are broken up in this direction.

Moreover, as illustrated in FIG. 4, the conductive materials 64 arepatterned or segmented in a y-direction, wherein the y-direction is adirection substantially perpendicular to an axis of the lead 60. Inother words, the conductive materials 64 are broken up in this directionsuch that one conductive material 64 is electrically isolated from aneighboring conductive material 64 in an immediate adjacent layer; i.e.,a conductive material 64 in the first patterned or segmented layer 80 iselectrically isolated from a neighboring conductive material 64 in thesecond patterned or segmented layer 82.

Preferably, the x-directional gap between non co-layer immediatelyx-directional adjacent conductive materials 64 is much smaller than thewavelength of the electromagnetic pulse that the shield must attenuateto prevent the incident pulse from passing unattenuated through theshield.

Lastly, the conductive materials 64 may be segmented in a z-direction,wherein the z-direction represents the planar surface of a layer as itis wrapped around the lead 60.

As noted above, the conductive material 64 is patterned or segmented tolimit the build up of eddy currents. Examples of the patterning of theconductive materials 64 are illustrated in FIGS. 5-7.

FIG. 5 illustrated a top view of a fabricated layer of the shielding. Asillustrated in FIG. 5, the conductive materials 64 are patterned orsegmented in such a way that a width of the conductive material 64 inthe x-direction, the x-direction being substantially parallel to an axisof a lead, is substantially equal. More specifically, the conductivematerials 64 is patterned or segmented in such a way that conductivematerials 64 spaced in a predetermined manner; e.g., the spacing betweenthe conductive materials 64, in the x-direction, may be equal or set ina predetermined manner. Lastly, the spacing between conductive materials64 in a z-direction, the z-direction representing the planar surface ofa layer as it is wrapped around a lead, is not necessarily equal or setin a predetermined manner; however the spacing in the z-direction may beequal or set in a predetermined manner.

FIG. 6 illustrated a top view of a fabricated layer of the shielding. Asillustrated in FIG. 6, the conductive materials 64 are patterned orsegmented in such a way that a width of the conductive material 64 inthe z-direction, the z-direction representing the planar surface of alayer as it is wrapped around a lead, is substantially equal. Morespecifically, the conductive materials 64 are patterned or segmented insuch a way that conductive materials 64 spaced in a predeterminedmanner; e.g., the spacing between the conductive materials 64, in thez-direction, may be equal or set in a predetermined manner. Lastly, thespacing between conductive materials 64 in a x-direction, thex-direction being substantially parallel to an axis of a lead, is notnecessarily equal or set in a predetermined manner; however the spacingin the x-direction may be equal or set in a predetermined manner.

FIG. 7 illustrated a top view of a fabricated layer of the shielding. Asillustrated in FIG. 7, the conductive material 64 are patterned orsegmented in such a way that the widths of the conductive materials 64in the z-direction, the z-direction representing the planar surface of alayer as it is wrapped around a lead, are random. More specifically, theconductive materials 64 are patterned or segmented in such a way thatconductive materials 64 are spaced in a random manner. Lastly, thespacing between conductive materials 64 in an x-direction, thex-direction being substantially parallel to an axis of a lead, is alsorandom.

It is noted that the although FIGS. 3-7 show a shielding with respect toa lead, the shielding can be used to shield other shaped devices such aselectrical component housings or other components of a medical assistdevice/system.

It is further noted that the actual dimensions of the non-conductivematerials can be set to suppress the radiation and eddy currents inducedfrom specific frequencies; e.g., the lengths of the non-conductivematerial can be varied to suppress or block specific or undesiredradiation frequencies. The dimensions of the non-conductive materialwould be on the order of the wavelength of the radiation so as tosuppress, block, or shield the radiation.

The actual layers may be formed using a photoresist/masking process suchas used in integrated circuit fabrication and thin film deposition. Byusing a mask/photoresist process, the conductive/insulative layers inthe vertical direction can be easily formed so that theconductive/insulative layers alternate (radially outward from the axisof the lead), and the conductive/insulative layers in the horizontaldirection also alternate (substantially parallel to an axis of thelead).

As noted above, the shielding may also be apertured. More specifically,implantable medical devices, such as pacemaker leads, typicallydemonstrate heating of 10-100° C., whereas body tissues can beirreversibly damaged by increases in temperature of as little as 2-4° C.Therefore, up to a 50× reduction in the amount of energy delivered tothe implanted medical device is required to make the device safe, 100×if a reasonable design margin of safety is desired. This reduction inthe amount of energy delivered to the implanted medical devicecorresponds to 20 dB of attenuation.

The performance of a shield or shield effectiveness (SE) with a circularopening has been shown to be:SE(dB)=20 Log[λ/2πr]where λ is the incident wavelength and r is the radius of the circularopening.

For holes of other shapes (e.g. an annulus) the factor 2πr may bereplaced by 2 L where L is the largest dimension of the opening (i.e.the circumference):SE(dB)=20 Log[λ/2L ]

This formula indicates that a slot or annulus with a greatest dimensionequal to 1/20 of the wavelength is required to provide a shieldingeffectiveness (SE) against heating effects of 20 dB.

The wavelength at a particular frequency in air is given by the equationλ=c/f, where c is the speed of light (3×10⁸ meters/second) and f is thefrequency. However, the wavelength in a particular material, such asbody tissues, is given by the following equation:λ_(m)=λ_(o)/[ε_(rel)]^(1/2)where ε_(rel) is the relative dielectric constant.

At a frequency of 64 MHz and ε_(rel)=80 (a typical value for bodytissue), λ_(o) is equal to 4.7 meters, and λ_(m) is equal to 0.5 meters.At a frequency of 64 MHz, a shield effectiveness of 20 dB requires thatL, the maximum aperture dimension (i.e. annulus circumference), notexceed 0.025 meters or 25 millimeters (L=0.5 meters/20).

However, it is well known that shielding effectiveness is reduced as thenumber of openings N is increased. Shield effectiveness scales as thesquare root on N.

In the present invention, N may range from 10 to 1000. Therefore, tomaintain minimum shield effectiveness against heating effects of 20 dB,the maximum aperture dimension should not exceed approximately 10millimeters to 1 millimeter, depending upon how many apertures oropenings are incorporated into the shield design. At higher frequenciessuch as 128 MHz (the frequency of emerging MRI facilities), the maximumaperture dimension should not exceed 5 millimeters to 0.5 millimeters.

Moreover, implantable medical devices, such as pacemaker leads, mustalso be capable of making sensitive electrical measurements such asintracardiac ECG signals, which are typically 1-25 millivolts. To insurethe accurate measurements of such signals, a measurement resolutioncapability of less than 0.1 millivolt is required. Experimentalmeasurements have shown that MRI procedures can induce voltages of asmuch as 100 volts in implanted pacing leads. Therefore, a shieldattenuation of 1,000,000 or 60 dB is required, which requires that themaximum aperture dimension must equal 1/2000 of the wavelength. At 64MHz, L=0.5 meters/2000=0.00025 meters or 0.25 millimeters. Since N mayrange from 10 to 1000, the maximum aperture dimension is further reducedto approximately 0.1 millimeters to 0.01 millimeters.

Therefore, depending upon the specific application and shield design, amaximum aperture dimension of 0.01 millimeters to 10 millimeters shouldbe used.

An example of such an apertured shielding for use with an electricallead is illustrated in FIG. 32. As shown in FIG. 32, an electrical lead60 is initially coated with an insulation layer 62 so as to electricallyinsulate the electrical lead from its surroundings. Upon the insulationlayer 62, a first patterned or apertured layer 800 of shielding isplaced or formed thereon. The first patterned or apertured layer 800includes conductive material 64 and non-conductive materials 66.

Upon the first patterned or apertured layer 800 of shielding, a secondpatterned or apertured layer 820 of shielding is placed or formedthereon. The second patterned or apertured layer 820 includes conductivematerial 64 and non-conductive materials 66.

It is noted that the conductive material 64 may be formed of a singleintegral piece of conductive material or be formed from a multitude ofpieces of conductive material, the multitude of pieces beingelectrically connected together in such a manner to function as a singleintegral piece of conductive material.

Upon the second patterned or apertured layer 820 of shielding, a thirdpatterned or apertured layer 840 of shielding is placed or formedthereon. The third patterned or apertured layer 840 includes conductivematerial 64 and non-conductive materials 66.

The conductive materials 64 of this embodiment of the present inventionmay be a metal, a carbon composite, nanotubes (wherein the nanotubes maybe constructed from a carbon base or the nanotubes could be formed fromother amalgams coated with the appropriate material(s)), metal-coatedcarbon filaments (wherein the metal may be one of the following metals:nickel, copper, cobalt, silver, gold, tin, or zinc), metal-coatedceramic filaments (wherein the metal may be one of the following metals:nickel, copper, cobalt, silver, gold, tin, or zinc), a composite ofmetal-coated carbon filaments and a polymer (wherein the polymer may beone of the following: polyether sulfone, silicone, polymide,polyvinylidene fluoride, or epoxy), a composite of metal-coated ceramicfilaments and a polymer (wherein the polymer may be one of thefollowing: polyether sulfone, silicone, polymide, polyvinylidenefluoride, or epoxy), a composite of metal-coated carbon filaments and aceramic (wherein the ceramic may be one of the following: cement,silicates, phosphates, silicon carbide, silicon nitride, aluminumnitride, or titanium diboride), a composite of metal-coated ceramicfilaments and a ceramic (wherein the ceramic may be one of thefollowing: cement, silicates, phosphates, silicon carbide, siliconnitride, aluminum nitride, or titanium diboride), or a composite ofmetal-coated (carbon or ceramic) filaments (wherein the metal may be oneof the following metals: nickel, copper, cobalt, silver, gold, tin, orzinc) and a polymer/ceramic combination (wherein the polymer may be oneof the following: polyether sulfone, silicone, polymide, polyvinylidenefluoride, or epoxy and the ceramic may be one of the following: cement,silicates, phosphates, silicon carbide, silicon nitride, aluminumnitride, or titanium diboride).

A more detailed description of the coated filaments is found in U.S.Pat. No. 5,827,997, entitled “Metal Filaments for ElectromagneticInterference Shielding.” The entire content of U.S. Pat. No. 5,827,997is hereby incorporated by reference.

The non-conductive materials 66 of this embodiment of the presentinvention may be a ceramic; glass; mica; anodized copper; metallicoxides; natural or synthetic rubbers; or resins, such as natural resins,epoxy resins, or silicones.

Over the shielding, a biocompatible layer 68 may be placed or formedthereon. Preferably, the biocompatible layer is a non-permeablediffusion resistant biocompatible material.

As illustrated in FIG. 32, the conductive material 64 is patterned orapertured in an x-direction through the utilization of non-conductivematerials 66, wherein the x-direction is a direction substantiallyparallel to an axis of the lead 60. In other words, the conductivematerial 64 is broken up, patterned, or apertured in this direction suchthat one non-conductive material 66 is physically separated from aneighboring non-conductive material 66 in the same layer.

Moreover, as illustrated in FIG. 32, the conductive materials 64 ofdifferent layers are patterned in a y-direction, wherein the y-directionis a direction substantially perpendicular to an axis of the lead 60. Inother words, the conductive materials 64 of different layers are brokenup in this direction such that one conductive material 64 iselectrically isolated from a neighboring conductive material 64 in animmediate adjacent layer; i.e., a conductive material 64 in the firstpatterned or apertured layer 800 is electrically isolated from aneighboring conductive material 64 in the second patterned or aperturedlayer 82.

Preferably, the distance of the overlap is much smaller than thewavelength of the electromagnetic pulse that the shield must attenuateto prevent the incident pulse from passing unattenuated through theshield.

If the length of the overlapping shield dimension is designed to exceedthe thickness of the shield material, the structure thus formedresembles a waveguide and frequencies well below its cut-off frequencyare subject to additional attenuation. For a frequency of 64-128 MHz,this overlap corresponds to approximately 2-3× the maximum aperturedimension.

Therefore, depending upon the specific application and shield design, aminimum shield overlap dimension should be 2-3× the maximum aperturedimension. It is also important to note that the heating may alsoincrease with increased frequency. At a static magnetic field of 1.5Tesla the RF transmission frequency is 63.7 MHz, at 3 T it is 127.5 MHz(factor of 42.5 MHz for each Tesla).

Lastly, the conductive material 64 may be segmented or apertured in az-direction, wherein the z-direction represents the planar surface of alayer as it is wrapped around the lead 60.

Another example of such a shielding for use with an electrical lead isillustrated in FIG. 33. As shown in FIG. 33, an electrical lead 60 isinitially coated with an insulation layer 62 so as to electricallyinsulate the electrical lead from its surroundings. Upon the insulationlayer 62, a first patterned or apertured layer 800 of shielding isplaced or formed thereon. The first patterned or apertured layer 800includes conductive material 64 and non-conductive materials 66.

It is noted that the conductive material 64 may be formed of a singleintegral piece of conductive material or be formed from a multitude ofpieces of conductive material, the multitude of pieces beingelectrically connected together in such a manner to function as a singleintegral piece of conductive material.

Upon the first patterned or apertured layer 800 of shielding, a layer.70of non-conductive material is placed or formed thereon. Upon the layer70 of non-conductive material, a second patterned or apertured layer 820of shielding is placed or formed thereon. The second patterned orapertured layer 820 includes conductive material 64 and non-conductivematerials 66.

The conductive materials 64 of this embodiment of the present inventionmay be a metal, a carbon composite, nanotubes (wherein the nanotubes maybe constructed from a carbon base or the nanotubes could be formed fromother amalgams coated with the appropriate material(s)), metal-coatedcarbon filaments (wherein the metal may be one of the following metals:nickel, copper, cobalt, silver, gold, tin, or zinc), metal-coatedceramic filaments (wherein the metal may be one of the following metals:nickel, copper, cobalt, silver, gold, tin, or zinc), a composite ofmetal-coated carbon filaments and a polymer (wherein the polymer may beone of the following: polyether sulfone, silicone, polymide,polyvinylidene fluoride, or epoxy), a composite of metal-coated ceramicfilaments and a polymer (wherein the polymer may be one of thefollowing: polyether sulfone, silicone, polymide, polyvinylidenefluoride, or epoxy), a composite of metal-coated carbon filaments and aceramic (wherein the ceramic may be one of the following: cement,silicates, phosphates, silicon carbide, silicon nitride, aluminumnitride, or titanium diboride), a composite of metal-coated ceramicfilaments and a ceramic (wherein the ceramic may be one of thefollowing: cement, silicates, phosphates, silicon carbide, siliconnitride, aluminum nitride, or titanium diboride), or a composite ofmetal-coated (carbon or ceramic) filaments (wherein the metal may be oneof the following metals: nickel, copper, cobalt, silver, gold, tin, orzinc) and a polymer/ceramic combination (wherein the polymer may be oneof the following: polyether sulfone, silicone, polymide, polyvinylidenefluoride, or epoxy and the ceramic may be one of the following: cement,silicates, phosphates, silicon carbide, silicon nitride, aluminumnitride, or titanium diboride).

Over the shielding, a biocompatible layer 68 may be placed or formedthereon. Preferably, the biocompatible layer is a non-permeablediffusion resistant biocompatible material.

The non-conductive materials 66 of this embodiment of the presentinvention may be a ceramic; glass; mica; anodized copper; metallicoxides; natural or synthetic rubbers; or resins, such as natural resins,epoxy resins, or silicones.

As illustrated in FIG. 4, the conductive materials 64 are patterned orapertured in an x-direction, wherein the x-direction is a directionsubstantially parallel to an axis of the lead 60. In other words, theconductive materials 64 are broken up, patterned, or apertured in thisdirection such that the non-conductive materials 66 break up theconductive material 64.

Moreover, as illustrated in FIG. 33, the conductive materials 64 arepatterned in a y-direction, wherein the y-direction is a directionsubstantially perpendicular to an axis of the lead 60. In other words,the conductive materials 64 are broken up in this direction such thatone conductive material 64 is electrically isolated from a neighboringconductive material 64 in an immediate adjacent layer; i.e., aconductive material 64 in the first patterned or apertured layer 800 iselectrically isolated from a neighboring conductive material 64 in thesecond patterned or apertured layer 820.

Preferably, the distance of the overlap is much smaller than thewavelength of the electromagnetic pulse that the shield must attenuateto prevent the incident pulse from passing unattenuated through theshield.

If the length of the overlapping shield dimension is designed to exceedthe thickness of the shield material, the structure thus formedresembles a waveguide and frequencies well below its cut-off frequencyare subject to additional attenuation. For a frequency of 64-128 MHz,this overlap corresponds to approximately 2-3× the maximum aperturedimension.

Therefore, depending upon the specific application and shield design, aminimum shield overlap dimension should be 2-3× the maximum aperturedimension.

Lastly, the conductive materials 64 may be segmented in a z-direction,wherein the z-direction represents the planar surface of a layer as itis wrapped around the lead 60.

As noted above, the conductive material 64 is patterned or apertured tolimit the build up of eddy currents. Examples of the patterning oraperturing of the conductive material 64 are illustrated in FIGS. 34-36.

FIG. 34 illustrated a top view of a fabricated layer of the shielding.As illustrated in FIG. 34, the conductive material 64 is patterned orapertured in such a way that a width of the conductive material 64 inthe x-direction, the x-direction being substantially parallel to an axisof a lead, is substantially equal. More specifically, the conductivematerial 64 is patterned or apertured in such a way that non-conductivematerials 66 spaced in a predetermined manner; e.g., the spacing betweenthe non-conductive materials 66, in the x-direction, may be equal or setin a predetermined manner. Lastly, the spacing between non-conductivematerials 66 in a z-direction, the z-direction representing the planarsurface of a layer as it is wrapped around a lead, is not necessarilyequal or set in a predetermined manner; however the spacing in thez-direction may be equal or set in a predetermined manner.

Preferably, the distance of the overlap is much smaller than thewavelength of the electromagnetic pulse that the shield must attenuateto prevent the incident pulse from passing unattenuated through theshield.

If the length of the overlapping shield dimension is designed to exceedthe thickness of the shield material, the structure thus formedresembles a waveguide and frequencies well below its cut-off frequencyare subject to additional attenuation. For a frequency of 64-128 MHz,this overlap corresponds to approximately 2-3× the maximum aperturedimension.

Therefore, depending upon the specific application and shield design, aminimum shield overlap dimension should be 2-3× the maximum aperturedimension.

FIG. 35 illustrated a top view of a fabricated layer of the shielding.As illustrated in FIG. 35, the conductive material 64 is patterned orapertured in such a way that a width of the conductive material 64 inthe z-direction, the z-direction representing the planar surface of alayer as it is wrapped around a lead, is substantially equal. Morespecifically, the conductive material 64 is patterned or apertured insuch a way that non-conductive materials 66 spaced in a predeterminedmanner; e.g., the spacing between the non-conductive materials 66, inthe z-direction, may be equal or set in a predetermined manner. Lastly,the spacing between non-conductive materials 66 in a x-direction, thex-direction being substantially parallel to an axis of a lead, is notnecessarily equal or set in a predetermined manner; however the spacingin the x-direction may be equal or set in a predetermined manner.

Preferably, the distance of the overlap is much smaller than thewavelength of the electromagnetic pulse that the shield must attenuateto prevent the incident pulse from passing unattenuated through theshield.

If the length of the overlapping shield dimension is designed to exceedthe thickness of the shield material, the structure thus formedresembles a waveguide and frequencies well below its cut-off frequencyare subject to additional attenuation. For a frequency of 64-128 MHz,this overlap corresponds to approximately 2-3× the maximum aperturedimension.

Therefore, depending upon the specific application and shield design, aminimum shield overlap dimension should be 2-3× the maximum aperturedimension.

FIG. 36 illustrated a top view of a fabricated layer of the shielding.As illustrated in FIG. 36, the conductive material 64 is patterned orapertured in such a way that the widths of the non-conductive materials66 in the z-direction, the z-direction representing the planar surfaceof a layer as it is wrapped around a lead, are random. Morespecifically, the conductive materials 64 are patterned or apertured insuch a way that non-conductive materials 66 spaced in a random manner.Lastly, the spacing between non-conductive materials 66 in anx-direction, the x-direction being substantially parallel to an axis ofa lead, is also random.

It is noted that the although FIGS. 32-36 show a patterned or aperturedshielding with respect to a lead, the patterned or apertured shieldingcan be used to shield other shaped devices such as electrical componenthousings or other components of a medical assist device/system.

It is further noted that the actual dimensions of the non-conductivematerials can be set to suppress the radiation and eddy currents inducedfrom specific frequencies; e.g., the lengths of the non-conductivematerial can be varied to suppress or block specific or undesiredradiation frequencies. The dimensions of the non-conductive materialwould be on the order of the wavelength of the radiation so as tosuppress, block, or shield the radiation.

Preferably, the distance of the overlap is much smaller than thewavelength of the electromagnetic pulse that the shield must attenuateto prevent the incident pulse from passing unattenuated through theshield.

The actual layers may be formed using a photoresist/masking process suchas used in integrated circuit fabrication and thin film deposition. Byusing a mask/photoresist process, the conductive/insulative layers inthe vertical direction can be easily formed so that theconductive/insulative layers alternate (radially outward from the axisof the lead), and the conductive/insulative layers in the horizontaldirection also alternate (substantially parallel to an axis of thelead).

Although, as stated above, eddy currents in a conductive structure maycause heating to the surrounding tissue, eddy currents induced in thetissue may also cause the heating of the surrounding tissue. As notedabove, when a substance such as human tissue is subjected to a staticmagnetic field, the individual magnetic moments of the spins in thetissue align in a parallel and anti-parallel direction with the staticmagnetic field. This direction along the static magnetic field can betermed as the longitudinal direction. In magnetic-resonance imaging, theradio frequency polarizing field used for spin manipulation isconstantly changing and thus, the individual magnetic moments of thespins in the tissue attempt to align with the polarizing field. Theconstant changing of alignment of the magnetic moments of the spins inthe tissue causes the tissue's temperature to increase, thereby exposingthe tissue to possible magnetic-resonance imaging induced thermaldamage. Thus, this second possible cause of undesirable heat needs toalso be addressed to prevent tissue damage caused by excessive heat.

In a further embodiment of the present invention, the heat beingaccumulated in the surrounding tissue is addressed and reduced orprevented, notwithstanding the source or cause of the excessive heat. Tobetter describe this embodiment, the term, specific absorption ratio,will be used in discussing the excessive heat.

The specific absorption ratio is defined as the amount of power absorbedby a sample within a magnetic-resonance imaging scanner. The samplecould be a suitable phantom mimicking tissue properties or a tissueregion of patient.

As noted above, this excessive heat could be caused by either eddycurrents induced in conductive structures or eddy currents induced inthe actual tissue. It is noted that although two distinct sources ofexcessive heat are identified for the purposes of describing thisembodiment of the present invention, the actual source of the heat maynot be known, nor is it a real consideration in the actual functionalityof this embodiment of the present invention.

With respect to induced eddy currents, due to their electromagneticreceptive capabilities, cardiac assist device leads, metallicguidewires, neurostimulator leads and intraluminal coil leads may inducea localized increase in the specific absorption ratio, thereby resultingin an increased heating of the surrounding tissue. Moreover, due to theindividual magnetic moments of the spins in the tissue attempting toalign with the polarizing field, the acquisition sequence of themagnetic-resonance imaging process may also induce a localized increasein the specific absorption ratio, thereby resulting in an increasedheating of the surrounding tissue. This tissue heating, notwithstandingthe source, may be a function of the time of radio-frequency excitation,gradient coil switching speed, or strength of the static magnetic field.

In this embodiment of the present invention, the localized specificabsorption ratio of tissue is reduced by altering the actualmagnetic-resonance imaging acquisition process. More specifically,according to a preferred embodiment of the present invention, thelocalized specific absorption ratio of tissue is reduced by varying theimaging pulse sequence and/or changing the timing parameters ofmagnetic-resonance imaging acquisition process. For example, in responseto increases in the localized specific absorption ratio or duringmagnetic-resonance image acquisition, the radio-frequency excitationcycle may be interrupted to allow for cooling of the tissue. In otherwords, if a threshold temperature level or a localized specificabsorption ratio is exceeded, the magnetic-resonance imaging acquisitioncan be halted until the tissue temperature or localized specificabsorption ratio decreases to an accepted level.

It is noted that the increased in the localized specific absorptionratio may or may not have resulted from placement of a conductivestructure near the tissue region.

FIG. 8 illustrates a magnetic-resonance imaging process according to theconcepts of the present invention, which alters the actualmagnetic-resonance imaging acquisition process in response to atemperature change in the tissue region being imaged. In this process,at Step S1, an image acquisition sequence is started by themagnetic-resonance imaging system. At step S2, the temperature change inthe tissue region being imaged is measured. It is noted that thistemperature change may be relative or absolute. The temperature changein the tissue region being imaged can be measured a number of ways.

For example, in one embodiment, the relative temperature change can bemonitored by magnetic image acquisition techniques. More specifically,relative temperature changes can be measured using a double gradientecho sequence to acquire a baseline phase image and a second phase imageat within a single radio-frequency excitation of one same pulsesequence. A relative spatial temperature map of the desired region ofinterest can be calculated based on the subtraction of the two phaseimages collected during the double gradient echo sequence acquisition.

A more detail description of this relative temperature changeacquisition process is described in U.S. Pat. No. 5,711,300, entitled“Real Time In Vivo Measurement Of Temperature Changes WithMagnetic-resonance Imaging.” The entire contents of U.S. Pat. No.5,711,300 are hereby incorporated by reference. It is noted that this isonly one example of collecting phase image data for relative temperaturemapping. Other methods for collecting phase image data for relativetemperature mapping may also be used.

It is further noted that absolute temperature changes can be determinedusing known methods, such as fiber optic thermocouples attached toguidewires or catheters, or non-contact thermographic imagingtechniques.

Upon calculating the temperature change of the desired tissue region,such as described above with respect to the relative spatial temperaturemap of the desired region of interest, at Step S3, a predetermined uppertemperature threshold is compared with the calculated temperaturechange, or in the example described above, compared with the relativetemperature map. If the temperature change exceeds or is equal to thepredetermined upper temperature threshold, the imaging acquisitionsequence is adjusted or halted so as to reduce the temperature changebelow the predetermined upper temperature threshold, at Step S4.

The process returns to Step S2 to make another determination of thetemperature change in the tissue region of interest. If the temperaturechange is below the predetermined upper temperature threshold, Step S5determines if the measured temperature change is above a predeterminedlower temperature threshold. If Step S5 determines that the measuredtemperature change is above a predetermined lower temperature threshold,the process returns to Step S4 and the imaging acquisition sequenceremains adjusted or halted so as to reduce the temperature of the tissueregion of interest. On the other hand, if Step S5 determines that themeasured temperature change is below or is equal to a predeterminedlower temperature threshold, the process resume image acquisition at toStep S6.

It is to be understood that this type of adjustment can be applied toany form of pulse sequence or imaging acquisition method so as it meetsthe requirements of the user. There are an infinite number of variationsin pulse sequence design and pulse sequence parameter determination tominimize heating. The actual variation in pulse sequence design andpulse sequence parameter determination will be dependent upon the imagequality and the temporal resolution of acquisition desired, however, therequired tissue temperature threshold(s) will be the ultimatedeterminant.

This embodiment may also include a user interactive process, which canbe programmed. An example of such a process is described in detail inU.S. Pat. No. 6,396,266, entitled “magnetic-resonance Imaging SystemWith Interactive magnetic-resonance Geometry Prescription Control.” Theentire contents of U.S. Pat. No. 6,396,266 are hereby incorporated byreference.

In another embodiment, the SAR coefficients of a wide range ofimplantable medical devices are programmed into the magnetic-resonanceimaging system, which then uses these data to predetermine and apply assafe magnetic-resonance imaging sequence.

In yet another embodiment, as illustrated in FIG. 9, amagnetic-resonance imaging process, according to the concepts of thepresent invention, alters the actual magnetic-resonance imagingacquisition process in response to a calculated specific absorptionratio of the tissue region being imaged. In this process, at Step S1, animage acquisition sequence is started by the magnetic-resonance imagingsystem. At step S21, the specific absorption ratio is estimated usingtheoretical modeling and/or empirical results from certain conductorsbeing imaged in tissues. At Step S31, the calculated specific absorptionratio level(s) are then be compared to threshold levels.

If the calculated specific absorption ratio level exceeds or is equal tothe predetermined upper specific absorption ratio level threshold, theimaging acquisition sequence is adjusted or halted so as to reduce thetemperature of the tissue region of interest, at Step S4.

The process returns to Step S21 to make another determination of thespecific absorption ratio level. If the determined specific absorptionratio level is below the predetermined upper specific absorption ratiolevel threshold, Step S51 determines if the determined specificabsorption ratio level is above a predetermined lower specificabsorption ratio threshold. If Step S51 determines that the determinedspecific absorption ratio level is above a predetermined lower specificabsorption ratio threshold, the process returns to Step S4 and theimaging acquisition sequence remains adjusted or halted so as to reducethe temperature of the tissue region of interest. On the other hand, ifStep S51 determines that the determined specific absorption ratio levelis below or is equal to a predetermined lower specific absorption ratiothreshold, the process resume image acquisition at to Step S6.

This embodiment may also include a user interactive process describedabove.

In the embodiments illustrated in FIGS. 8 and 9, specific absorptionratio or energy deposition of radio frequency energy in the tissue canbe reduced by specific manipulations of each individual radio frequencypulse. Moreover, these embodiments of the present invention maymanipulate the pulse waveform, pulse duration and pulse amplitude toreduce the specific absorption ratio. The choice of the integratedvalues of these three parameters is determined upon by the requirementsof the user. For example, reduction of the amplitude and increase in theduration of the pulse may reduce the specific absorption ratio, but thismay not be appropriate for the specified timing required to imagecertain contrast parameters.

A more detail discussion of the specific manipulations that be used toreduce specific absorption ratio is set forth in U.S. Pat. No.4,760,336, entitled “Variable Rate Magnetic Resonance SelectiveExcitation For Reducing radio-frequency Power And Specific AbsorptionRate.” The entire contents of U.S. Pat. No. 4,760,336 are herebyincorporated by reference.

It is also noted that the embodiments illustrated in FIGS. 8 and 9 canbe used not only as a stand-alone solution to reduce the increases inspecific absorption ratio resulting from conductive structures intissue, but also in conjunction with the patterned shielding embodimentsdescribed above.

A block diagram of an ECG acquisition and processing system is shown inFIG. 10. The ECG monitoring system can be used as a stand-alone ECGmonitor or as part of an external or internal pacemaker. A patient isconnected to ECG monitoring circuitry 110 via surface or internalelectrodes 112 and lead wires 114. The lead wires 114 connect to theinput ECG/radio-frequency filter 116.

This filter design enables the monitoring circuit to operate in the highradio-frequency field environment generated by the magnetic-resonanceimaging system. First, the radio-frequency filter prevents thedevelopment of excess voltage on the ECG input amplifier and processor118 that would otherwise damage the amplifier, and excess voltage on theECG leads potentially inducing a dangerous heart rhythm. Second, itprevents the development of excess current on the ECG electrodes 112that would induce thermogenic damage at the electrode-heart tissueinterface that could lead to loss of capture of pacing signals andpossibly death. Third, the radio-frequency filter 116, when attachedbetween the lead wires 114 and the ECG input amplifier, will notinterfere with proper sensing of the ECG signal.

The input ECG/radio-frequency filter 116 has the characteristics of alow-pass filter with approximately 100 dB signal attenuation in thecommonly used magnetic-resonance imaging system radio-frequencyfrequency range. The frequencies contained in the intracardiac orsurface ECG signal, however, are passed without any attenuation.

In the case where the lead wires 114 are of long length, such as inexternal monitoring, there is significant stray capacitance between thelead wires. As a result, electromagnetic fields generated between theelectrodes 112 can produce dangerously high voltage within and currentflow through the electrodes.

To limit this voltage and current, inductors 120, may be placed in thelead wires 114, close to the electrodes. The ECG/radio-frequency inputfilter 116 and lead wire inductors 120 must contain components with aresonance frequency higher than the radio-frequency frequency of themagnetic-resonance imaging system in use.

The ECG amplifier/processor 118 contains low-pass or band reject filter119 placed after the initial amplifier 117 and before final stages 121.The low-pass or band reject filter must pass the QRS signals and rejectgradient field and radio-frequency frequencies. The output ECG signal122, which can take any conventional form such as an R-wave detectoroutput or analog ECG, can be used for any purpose. It can be used todisplay the ECG on monitor 124, or synchronize an external or internaldevice 126, such as a magnetic-resonance imaging system or cardiacpacemaker.

In another embodiment shown in FIG. 11, a pacemaker system 128 isdescribed. This pacemaker system can be either external or implantable.In addition to the elements shown in FIG. 10, this embodiment alsocontains a pacing output stage 130 that is controlled by the pacinglogic 132.

The pacing logic is itself controlled by signals from the ECGamplifier/processor 134. The patient is connected to the pacemakersystem 128, via surface or internal sensing electrodes 136 and leadwires 138. The lead wires are connected to the ECG amplifier/processor134, via the input ECG/radio-frequency filter 140. The pacing outputstage 130 is connected to pacing electrodes 142 via the outputradio-frequency filter 144 and pacing lead wires 146. The ECGamplifier/processor 134 contains a low-pass or band reject filter 135placed after the initial amplifier 133 and before the final stages 137.

The low-pass or band reject filter must pass the QRS signal and rejectgradient field frequencies caused by the magnetic-resonance imaging. Asin the previous embodiment, inductors 148, 150 may be placed in the leadwires, 138, 146 close to the electrodes 136, 142. In this embodiment,the sensing 136 and pacing 142 electrodes, wires 138, 146, inductors148, 150, and radio-frequency filters 149, 144 are shown separately.However, these components can be merged to create a combined ECGsensing/pacing radio-frequency electrodes, wires, inductors, andfilters.

An implantable pacemaker 150 capable of bi-directional communicationwith an external programmer 152 is shown in FIG. 12. The implantableunit contains conventional programmable pacing logic 154 for sensingand/or pacing the ventricle and/or atrium and includes an ECG amplifierand processor 156, pacing output stage 158, combined input/outputradio-frequency filter 160, electrodes 162 and leads 164. The ECGamplifier and processor 156 contains a low-pass or band reject filter157 placed after the initial amplifier 155 and before final stages 159.The low-pass or band reject filter must pass the QRS signals and rejectgradient field frequencies caused by the magnetic-resonance imaging.

As in the previous embodiment, inductors 166 may be placed in the leadwires 164 close to the electrodes 162. The electrodes 162, leads 164,inductors 166, and input/output filter 160 may be single elementsperforming combined input and output functions or may be separate inputand output elements. For dual chamber pacemakers, multiple sets ofsensing/pacing electrodes may be used 159, 162, which are each connectedto input/output radio-frequency filters 160.

A first electrode pair may be for atrium sensing or pacing and a secondelectrode pair may be for ventricle sensing or pacing. As is taught inthe pacing art, pacing logic 154 may be programmed, for example, tosense the atrium signal and pace via the ventricle electrodes. Inaddition, a new form of circulatory support called cardiomyoplasty mayutilize other electrodes 161 and leads 167 to pace skeletal musclewrapped around the heart and stimulated in synchrony with pacing theheart. In each type of pacemaker the leads 163, 164, 167 may containinductors 165, 166, 169 to reduce electrical currents across theelectrodes and each lead may be connected to separate input/outputradio-frequency filter 160.

In order to protect the pacemaker from the effects of theradio-frequency field generated by the magnetic-resonance imagingsystem, it is necessary to surround the components with aradio-frequency shield 168. In a preferred embodiment of the presentinvention, this shield is as described above with respect to FIGS. 3through 7.

More specifically, the shield may consist of patterned layers includingconductive materials and non-conductive materials. The conductivematerials of the present invention may be a metal, a carbon composite,nanotubes (wherein the nanotubes may be constructed from a carbon baseor the nanotubes could be formed from other amalgams coated with theappropriate material(s)), metal-coated carbon filaments (wherein themetal may be one of the following metals: nickel, copper, cobalt,silver, gold, tin, or zinc), metal-coated ceramic filaments (wherein themetal may be one of the following metals: nickel, copper, cobalt,silver, gold, tin, or zinc), a composite of metal-coated carbonfilaments and a polymer (wherein the polymer may be one of thefollowing: polyether sulfone, silicone, polymide, polyvinylidenefluoride, or epoxy), a composite of metal-coated ceramic filaments and apolymer (wherein the polymer may be one of the following: polyethersulfone, silicone, polymide, polyvinylidene fluoride, or epoxy), acomposite of metal-coated carbon filaments and a ceramic (wherein theceramic may be one of the following: cement, silicates, phosphates,silicon carbide, silicon nitride, aluminum nitride, or titaniumdiboride), a composite of metal-coated ceramic filaments and a ceramic(wherein the ceramic may be one of the following: cement, silicates,phosphates, silicon carbide, silicon nitride, aluminum nitride, ortitanium diboride), or a composite of metal-coated (carbon or ceramic)filaments (wherein the metal may be one of the following metals: nickel,copper, cobalt, silver, gold, tin, or zinc) and a polymer/ceramiccombination (wherein the polymer may be one of the following: polyethersulfone, silicone, polymide, polyvinylidene fluoride, or epoxy and theceramic may be one of the following: cement, silicates, phosphates,silicon carbide, silicon nitride, aluminum nitride, or titaniumdiboride).

Since the implantable pacemaker shown in FIG. 12 is enclosed in aradio-frequency shield, it was necessary to develop a unique method ofexternal programming. The information received by the pacemaker 150 isprocessed to separate the desired communication signal from theradio-frequency signals produced by the magnetic-resonance imagingsystem. An antenna 170 is connected to the telemetry radio-frequencyamplifier 172 via the telemetry receiver/band pass filter 174. Thisfilter is a band pass filter that passes only the specific programmingfrequency to the telemetry amplifier 172.

The telemetry logic circuit 176 interprets all radio-frequency receivedby the telemetry antenna 170 and will only allow programming of thepacing logic circuit 154 when a specific telemetry enable pattern isreceived. The telemetry enable logic circuit 176 will also inhibitcontrol of the pacing output stage 158 while programming is takingplace. This prevents improper and potentially dangerous pacingparameters from controlling the pacing stage. As a further safetymeasure, the telemetry enable circuit 176 enables pacing at a presetrate (“safety pacing”) to provide adequate pacing back-up for pacemakerdependent patients during external programming.

FIGS. 13 through 18 are schematic diagrams of the inductor elements(elements 120, 148, 150, 165, 166, and 169) that are placed in the leadwires to reduce harmful currents. As mentioned previously, significantstray capacitance can be produced along the lead wires, particularly forexternal pacers. As a result,

Electromagnetic fields generated between the electrodes because of theradio-frequency field can produce dangerously high voltages across andcurrents flowing through the electrodes. To limit this current, andenhance patient safety, the inductor elements shown in FIGS. 13 through18 are placed in the lead wires close to the electrodes. FIG. 13 showsthe inductor elements 178 when a two-wire lead is used. FIG. 14 showsthe inductor elements 180 when a two-wire shielded lead is used. FIG. 15shows alternative inductor elements 182, which incorporate a capacitor184, thus comprising an additional low-pass L-C filter. FIG. 16 showsthe same alternative inductor elements 183 and a capacitor 185, as shownin FIG. 15, but using a shielded two-wire lead. FIG. 17 shows theinductor elements 186 used in a multi-lead shielded harness. FIG. 18shows alternative inductor elements 188 used in a multi-lead shieldedharness, which also includes capacitor components 190 connected to theshielding enclosure of the capacitors.

The leads can be used to measure ECG, EEG, or other electrical signalsof physiological significance. FIGS. 13 through 18 show twisted leads,but it is to be understood that coaxial as well as other forms of leadwires could be used.

FIGS. 19 through 31 show various embodiments of the inputradio-frequency filter, output radio-frequency filter, and combinedinput/output radio-frequency filter. According to the concepts of thepresent invention, there are many possible specific implementations ofthe radio-frequency filter design that will depend on: 1) whether twoleads or multiple leads are used; 2) whether the leads are shielded; 3)whether single or multiple stage filtering is used; and 4) whether eachradio-frequency filter is housed in a separate shielded enclosure.

It should be apparent that the filters could be placed either at theproximal end or the distal end of the pulse generator—lead assembly.

It will also become apparent that the same filter design principles canbe used whether the filter is acting as an input filter for an ECGmonitor, as separate input filters and output filters for a pacemaker,or as a single combined input/output filter for a pacemaker.

FIGS. 19 and 20 show one-stage filters for use with two unshieldedleads. FIGS. 21 and 22 show alternative ways to shield the electronics.

FIG. 19 shows the input filter 192 in a separate shielded enclosure;whereas FIG. 20 shows the input filter 196 located in the same-shieldedenclosure 198 as the processing electronics 1100 with an internal wall1102 shielding the monitoring electronics 1100 from the input filter196. (Whether one or two enclosures are used, the filter section shouldbe shielded from the other circuitry.)

As can be seen in FIG. 19, each lead is connected to a separate low-passfilter. (Leads 1103 may be twisted to reduce electromagneticinterference). A first low-pass filter is made from inductor 1104 andcapacitor 1106; and a second low-pass filter is made from inductor 1108and capacitor 1110. Each low-pass filter offers approximately 100 dBsignal attenuation in the radio-frequency frequency range of themagnetic-resonance imaging system and passes with little attenuation thedesired physiological signal.

As mentioned previously, the L-C components must be high frequencyelements retaining their desired characteristics throughout thefrequency range. The output from the input filter 192 is connected todifferential amplifier 1112. In order to properly utilize thedifferential amplifier 1112, the non-inverting input of the ECGamplifier 1112 is connected to the amplifier zero-signal referenceterminal via a resistor 1114, as is well known in the art.

Obviously, any other method of converting a two to three conductor inputmay also be used. The two capacitors 1106, 1110 in the low-pass filtersare referenced to a third wire 1116 that is connected to the shieldsurrounding the filter 1122 and also connected to the zero-signalreference terminal of the ECG amplifier 1112.

The ECG amplifier may be a single differential amplifier or acombination of multiple differential amplifiers. Alternatively,capacitors 1106, 1110 can be connected directly to the shield 1122 witha separate wire connecting the shield 1122 to the zero-signal referenceterminal of the ECG amplifier 1112.

To improve the characteristics of the filter, resistors 1118, 1120 maybe added between the input filter 192 and the input of the differentialamplifier 1112. Just as the input filter 192 is enclosed in a shield1122, the ECG amplifier 1112 and processor 1124 may also be enclosed ina separate shield 1125.

The shields may consist of patterned layers including conductivematerials and non-conductive materials. The conductive materials of thepresent invention may be a metal, a carbon composite, nanotubes (whereinthe nanotubes may be constructed from a carbon base or the nanotubescould be formed from other amalgams coated with the appropriatematerial(s)), metal-coated carbon filaments (wherein the metal may beone of the following metals: nickel, copper, cobalt, silver, gold, tin,or zinc), metal-coated ceramic filaments (wherein the metal may be oneof the following metals: nickel, copper, cobalt, silver, gold, tin, orzinc), a composite of metal-coated carbon filaments and a polymer(wherein the polymer may be one of the following: polyether sulfone,silicone, polymide, polyvinylidene fluoride, or epoxy), a composite ofmetal-coated ceramic filaments and a polymer (wherein the polymer may beone of the following: polyether sulfone, silicone, polymide,polyvinylidene fluoride, or epoxy), a composite of metal-coated carbonfilaments and a ceramic (wherein the ceramic may be one of thefollowing: cement, silicates, phosphates, silicon carbide, siliconnitride, aluminum nitride, or titanium diboride), a composite ofmetal-coated ceramic filaments and a ceramic (wherein the ceramic may beone of the following: cement, silicates, phosphates, silicon carbide,silicon nitride, aluminum nitride, or titanium diboride), or a compositeof metal-coated (carbon or ceramic) filaments (wherein the metal may beone of the following metals: nickel, copper, cobalt, silver, gold, tin,or zinc) and a polymer/ceramic combination (wherein the polymer may beone of the following: polyether sulfone, silicone, polymide,polyvinylidene fluoride, or epoxy and the ceramic may be one of thefollowing: cement, silicates, phosphates, silicon carbide, siliconnitride, aluminum nitride, or titanium diboride).

FIG. 20 shows an alternative embodiment where the input filter 196 andprocessing electronics 1100 are housed in a single contiguous shieldingenclosure 198. A separate metallic wall 1102 is used to shield theelectronics 1100 from the input filter 196. In this embodiment, the twocapacitors 1126, 1128 that make up the low-pass filters are connected tothe zero-signal reference terminal of the differential amplifiers 1130via a third reference wire 1132; the reference wire. 1132 andzero-signal reference terminal 1134 both connected to the pacemakershielding case at a single point.

Both FIGS. 19 and 20 show processing electronics 1224, 1136 that areused to further process the signal obtained from the differentialamplifiers 1112, 1130. The processing electronics may be used to processand display the ECG signal, in the case of an external monitor, or beused to process and control pacing in the case of a pacemaker. It is tobe understood that the filter embodiments shown in FIGS. 19 and 20 couldbe used in a device that monitors physiologically significant electricalsignals other than ECG.

FIGS. 21 and 22 show one-stage filter designs 1135 and 1137 that ispreferred when the input leads are shielded. The design is identical tothat shown in FIGS. 19 and 20, except that the lead shield must becoupled to the shield enclosure via a radio-frequency filter. Toaccomplish this, the lead shield 1140 of the leads 1138, in FIG. 21, isconnected by an inductor 1146 to the third reference wire 1148, and thereference wire is connected to the filter's shielding enclosure 1150.Analogously, the shield 1144 of the leads 1142 in FIG. 22 is connectedby an inductor 1154 to the reference wire 1156, connected in turn to theenclosure 1160. In order to further reduce the effects of themagnetic-resonance imaging system radio-frequency signal, more than onestage of filtering can be used.

In the embodiment shown in FIG. 23, two stages of filtering are used.Each filter stage is similar to the filter described in FIG. 19. Eachinput lead is connected to a low-pass filter. Each low-pass L-C filterhas a capacitor connected to a reference wire that is connected to theshielding enclosure.

The first stage filter 1162 contains low-pass filters comprisinginductors 1164, 1166 and capacitors 1168, 1170. Capacitors 1168, 1170are coupled to a reference wire 1172 that is connected to the enclosure1174. Similarly, the second filter stage 1176 contains low-pass filtersmade from inductors 1178, 1180 and capacitors 1182, 1184, withcapacitors 1182, 1184 coupled to the reference wire 1172 that isconnected to the shielding enclosure 1177. All the capacitors (1168,1170, 1182 and 1184) used in the low-pass filters are connected to thereference wire 1172 which is connected to the zero-signal referenceterminal of the differential amplifier 1188.

Each stage in FIG. 23 is housed in a separate shielding enclosure andeach shielding enclosure is coupled to the reference wire 1172. It isunderstood that this concept can be extended to multistage filtering.The second stage 1176 could actually represent the Nth stage of amultistage filter, with each identical stage coupled together as shown.

An alternative embodiment for multiple stage filtering is shown in FIG.24. In this embodiment each filtering stage as well as the differentialamplifier and the associated electronics are housed in the samecontiguous shielding enclosure 1192. The filter design in thisembodiment is identical to that shown in FIG. 20; however, the referencewire is only connected to the shielding case 1192 at a single point. Ineffect, the low-pass filters in each stage and the zero-signal referenceterminal of the differential amplifier 1194 are all referred to the samepoint on the shielding enclosure. It is to be understood that theembodiment shown in FIG. 24 may also have three or more stages, eachstage shielded and coupled together as shown.

FIGS. 25 and 25 are schematic drawings of a two-stage filter for usewith shielded leads. These filters are similar to the multistage filterembodiments shown in FIGS. 23 and 24, except that the lead shields arecoupled via inductors to the reference wire in the last filter stage.FIG. 25 is a two-stage filter where each stage is separately isolated inenclosures 1196, 1198. Again, as in the previous embodiments, low-passfilters are connected in each input lead and referenced to a commonreference line 1200 (i.e., capacitors 1202, 1204, 1206, and 1208 areconnected to the reference line 1200).

The common reference line 1200 is connected to the shielding enclosurefor each stage and is input to the zero-signal reference terminal of thedifferential amplifier 1210.

The lead shield 1212 is connected to inductor 1214 in the first filterstage and via line 1215 to inductor 1216 in the second filter stage. Theinductor 1216 in the second, or final filter stage, is connected to thecommon reference line 1290. As discussed previously, the inductors 1214,1216 provide high impedance to the undesirable high frequencyradio-frequency voltages produced by the magnetic-resonance imagingsystem and prevent introduction of radio-frequency interference into theprocessing electronics via the common wire.

To increase performance, an additional capacitor 1218 may be connectedbetween lines 1215 and 1200 in the first stage and any subsequent filterstage other than the last stage. Inductor 1214, coupled with capacitor1218, provides a third low-pass filter.

FIG. 26 is a multistage filter housed in a single shielding enclosure1220 with separate walls 1222, 1224 isolating the filtering stages fromeach other and from the processing electronics. In the first stage 1226,each lead 1228 and the lead shield 1230 are coupled to low-pass filtersreferenced by capacitors 1230, 1232, and 1234 to a common reference line1236. In the last stage 1238, only the two lead wires contain low-passfilters referenced by capacitors 1240, 1242 to the common reference line1236. The lead shield 1230 is coupled via inductor 1244 directly to thereference line 1236.

As in the other embodiments, the reference line 1236 is connected to thezero-signal reference terminal of the differential amplifier 1248. Alsoas in the other embodiments, when a single case is used, the commonreference line 1236 is connected to the shielding enclosure at a singlepoint. It is of course understood that the two-stage filters shown inFIGS. 25 and 26 could be easily extended to multiple stage filteringwith each stage connected as described.

FIGS. 27 and 28 show an extension of the single and multiple stagefiltering to multiple lead shielded ECG harness. In FIG. 27, a fiveconductor shielded cable 1250 is filtered by a single stage filter 1252.In FIG. 28, a five conductor shielded cable 1254 is filtered by amultiple stage filter 1256.

Thus far, the filters have been described as input filters. However, asshown in FIG. 29, the filters can operate as both input and outputfilters. As generally true in pacemakers, the lead pair 1258 can be usedto sense electrical activity of the heart as well as to pace the heart.As shown previously, low pass filters (i.e., inductor 1260/capacitor1262 pair and inductor 1264/capacitor 1266 pair) for each lead arereferenced to a common reference line 1268. The reference line 1268 thenconnects to the zero-signal reference terminal of the differentialamplifier and to the shielding enclosure. A pacing amplifier 1270 incontrolled by electronics 1272 to generate a pacing signal. The pacingsignal is connected via lines 1274, 1276 through the filter, to theleads 1258. Two inductors 1278, 1280 are placed in lines 1274, 1276 tofurther prevent any current generated by the magnetic-resonance imagingsystem from affecting the pacing amplifier 1270; the inductors 1278, 1provide an additional high impedance to the high frequencyradio-frequency signals produced by the magnetic-resonance imagingsystem.

Connecting the pacing amplifier to the leads 1258 via the filter can beused with each of the filter embodiments described above. In multistagefiltering embodiments, pacing amplifiers would be preferably connectedinto the last filtering stage.

FIG. 30 is an exemplary embodiment, showing how the pacing amplifier1282 can be connected via a multi-stage filter to the shieldedpacing/sensing leads 1234. The pacing amplifier 1282 is connected viainductors 1286 and 1288 to the lead wires in the last stage of thefilter. As discussed above, the inductors 1286, 1288 provide additionalprotection from the high frequency radio-frequency signals generated bythe magnetic-resonance imaging system.

There are an unlimited number of ways in which the filters describedherein can be connected to both sensing electronic and pacingamplifiers. In fact, the same filter design can be used in stimulators,as shown in FIG. 31, where only a pacing signal is produced.

The filter design is identical to those described previously. The leads1290 are each connected to low pass filters (inductor 1291/capacitor1292 pair and inductor 1293/capacitor 1294 pair) that are referred bycapacitors 1292, 1294 to a common reference line 1296. Inductors 1298,1300 provide additional isolation to protect the pacing amplifier. Thefilter design for stimulation use only can be extended to multistageembodiments as described above.

In order to protect the implantable pacemaker elements from the effectsof the radio-frequency signals produced by the magnetic-resonanceimaging system, it is necessary to surround the components with aradio-frequency shield.

This shielding may consist of patterned layers including conductivematerials and non-conductive materials. The conductive materials of thepresent invention may be a metal, a carbon composite, nanotubes (whereinthe nanotubes may be constructed from a carbon base or the nanotubescould be formed from other amalgams coated with the appropriatematerial(s)), metal-coated carbon filaments (wherein the metal may beone of the following metals: nickel, copper, cobalt, silver, gold, tin,or zinc), metal-coated ceramic filaments (wherein the metal may be oneof the following metals: nickel, copper, cobalt, silver, gold, tin, orzinc), a composite of metal-coated carbon filaments and a polymer(wherein the polymer may be one of the following: polyether sulfone,silicone, polymide, polyvinylidene fluoride, or epoxy), a composite ofmetal-coated ceramic filaments and a polymer (wherein the polymer may beone of the following: polyether sulfone, silicone, polymide,polyvinylidene fluoride, or epoxy), a composite of metal-coated carbonfilaments and a ceramic (wherein the ceramic may be one of thefollowing: cement, silicates, phosphates, silicon carbide, siliconnitride, aluminum nitride, or titanium diboride), a composite ofmetal-coated ceramic filaments and a ceramic (wherein the ceramic may beone of the following: cement, silicates, phosphates, silicon carbide,silicon nitride, aluminum nitride, or titanium diboride), or a compositeof metal-coated (carbon or ceramic) filaments (wherein the metal may beone of the following metals: nickel, copper, cobalt, silver, gold, tin,or zinc) and a polymer/ceramic combination (wherein the polymer may beone of the following: polyether sulfone, silicone, polymide,polyvinylidene fluoride, or epoxy and the ceramic may be one of thefollowing: cement, silicates, phosphates, silicon carbide, siliconnitride, aluminum nitride, or titanium diboride).

In the various embodiments described above with respect to FIG. 10through 31, a common reference line was described in the context of aunipolar lead system. It is noted that the common reference line may notbe a separate line, but the common reference line may be one of theleads in a bipolar lead system. Moreover, the common reference line maya type of ground path (or line) or signal return path (or line). Thecommon reference line merely provides electrical ground for the filters.

In summary the present invention, as described above, is anelectromagnetic shield having a first patterned layer havingnon-conductive materials and conductive materials and a second patternedlayer having non-conductive materials and conductive materials. Theconductive materials in the first patterned layer may be randomlylocated or located in a predetermined segmented pattern. Moreover, theconductive materials in the first patterned layer may be located in apredetermined segmented pattern with respect to locations of theconductive materials in the second patterned layer. The electromagneticshield may further include a non-conductive layer between the first andsecond patterned layers such that the conductive materials in the firstpatterned layer can overlap the conductive materials in the secondpatterned layer with respect to a direction substantially perpendicularto a planar surface of the second patterned layer.

Therefore, the present invention provides a patterned shielding thatsubstantially prevents undesired radiation from entering the lead fromthe radial direction or electrical component and also substantiallyprevents the formation or inducement of eddy currents on the surface ofthe shielding, which are induced by the pulsing of the radiation orchanging magnetic field of the radiation. More specifically, theconductive material in a shielding layers of the present invention ispatterned or segmented so that the conductive material is literallybroken up to limit the build up of eddy currents induced by the changingmagnetic fields.

Moreover, the present invention can reduce specific absorption ratio byaltering the magnetic-resonance imaging acquisition process by alteringthe imaging pulse sequence and/or changing the timing parameters.Furthermore, the radio-frequency excitation cycle may be interrupted toallow for cooling. Therefore if a threshold temperature level isexceeded, the imaging acquisition can be halted until the tissuetemperature is at an acceptable level.

While various examples and embodiments of the present invention havebeen shown and described, it will be appreciated by those skilled in theart that the spirit and scope of the present invention are not limitedto the specific description and drawings herein, but extend to variousmodifications and changes all as set forth in the following claims.

1. A medical assist system, comprising: a primary device housing; saidprimary device housing having a control circuit therein; a lead systemto provide an electrical path between a tissue region and said primarydevice housing; and a shielding formed around said lead system to shieldsaid lead system from electromagnetic interference; said shielding beingformed of a first layer patterned with non-conductive materials andconductive materials and a second layer patterned with non-conductivematerials and conductive materials; wherein the conductive materials ofthe first layer are patterned in a direction substantially parallel toan axis of the lead system such that each of the conductive materials isphysically separated from the other conductive materials of the firstlayer; wherein the conductive materials of the second layer arepatterned in a direction substantially parallel to an axis of the leadsystem such that each of the conductive materials is physicallyseparated from the other conductive materials of the second layer; andwherein the conductive materials of the first layer and the conductivematerials of the second layer are patterned in a direction substantiallyperpendicular to the axis of the lead system such that each of theconductive materials of the first layer is electrically isolated fromthe conductive materials in the second layer.
 2. The medical assistsystem as claimed in claim 1, wherein said conductive material is ametal to shield said lead system from electromagnetic interference. 3.The medical assist system as claimed in claim 1, wherein said conductivematerial includes metal-coated carbon filaments to shield said leadsystem from electromagnetic interference.
 4. The medical assist systemas claimed in claim 1, wherein said conductive material is a polymercomposite to shield said lead system from electromagnetic interference.5. The medical assist system as claimed in claim 1, further comprising anon-permeable diffusion resistant biocompatible material covering saidshielding.
 6. The medical assist system as claimed in claim 1, whereinsaid shielding is formed around said primary housing to shield saidprimary housing from electromagnetic interference.
 7. The medical assistsystem as claimed in claim 1, wherein said first and second layers areconstructed of first and second patterned sheaths, each patterned sheathhaving non-conductive materials and conductive materials.
 8. The medicalassist system as claimed in claim 7, wherein said non-conductivematerials in one of said first and second patterned sheaths are randomlylocated.
 9. The medical assist system as claimed in claim 7, whereinsaid non-conductive materials in said first and second patterned sheathsare located in a predetermined segmented pattern.
 10. The medical assistsystem as claimed in claim 7, wherein said non-conductive materials insaid first and second patterned sheaths are located in a predeterminedsegmented pattern with respect to a direction substantially parallelwith an axis of said lead system.
 11. The medical assist system asclaimed in claim 10, wherein said non-conductive materials in said firstand second patterned sheaths are located in a predetermined segmentedpattern with respect to a direction substantially perpendicular with anaxis of said lead system.
 12. The medical assist system as claimed inclaim 7, wherein said non-conductive materials in said first and secondpatterned sheaths are located in a predetermined segmented pattern withrespect to a direction substantially perpendicular with an axis of saidlead system.
 13. The medical assist system as claimed in claim 1,wherein said lead system includes an electrical lead.
 14. The medicalassist system as claimed in claim 13, wherein said electrical lead isone of a bipolar lead and a unipolar lead.
 15. The medical assist systemas claimed in claim 13, wherein said lead system includes a combinationof unipolar and bipolar leads.
 16. The medical assist system as claimedin claim in 13, wherein said lead system includes an insulator layerbetween said electrical lead and said shielding.
 17. The medical assistsystem as claimed in claim 1, wherein said lead system includes anintraluminal coil.
 18. The medical assist system as claimed in claim 1,wherein said lead system includes a catheter guide wire.
 19. The medicalassist system as claimed in claim 1, wherein said lead system includes alead for a neurostimulation probe.
 20. The medical assist system ofclaim 1, wherein the first layer is immediately adjacent to the secondlayer.
 21. A medical assist device, comprising: a primary devicehousing; and said primary device housing having a control circuittherein; and a shielding formed around said primary device housing toshield said primary device housing and any circuits therein fromelectromagnetic interference; said shielding being formed of a firstlayer patterned with non-conductive materials and conductive and asecond layer patterned with non-conductive materials and conductivematerial; wherein the conductive materials of the first layer arepatterned in a direction substantially parallel to an axis of the leadsystem such that each of the conductive materials is physicallyseparated from the other conductive materials of the first layer;wherein the conductive materials of the second layer are patterned in adirection substantially parallel to an axis of the lead system such thateach of the conductive materials is physically separated from the otherconductive materials of the second layer; and wherein the conductivematerials of the first layer and the conductive materials of the secondlayer are patterned in a direction substantially perpendicular to theaxis of the lead system such that each of the conductive materials ofthe first layer is electrically isolated from the conductive materialsin the second layer.
 22. The medical assist device as claimed in claim21, wherein said conductive material is a metal to shield said primaryhousing from electromagnetic interference.
 23. The medical assist deviceas claimed in claim 21, wherein said conductive material includesmetal-coated carbon filaments to shield said primary housing fromelectromagnetic interference.
 24. The medical assist device as claimedin claim 21, wherein said conductive material is a polymer composite toshield said primary housing from electromagnetic interference.
 25. Themedical assist device as claimed in claim 21, further comprising a non-permeable diffusion resistant biocompatible material covering saidshielding.
 26. The medical assist system as claimed in claim 21, whereinsaid conductive material comprises nanotubes to shield said lead systemfrom electromagnetic interference.
 27. The medical assist device asclaimed in claim 21, wherein said conductive material comprisesnanotubes to shield said primary housing from electromagneticinterference.
 28. The medical assist system of claim 21, wherein thefirst layer is immediately adjacent to the second layer.
 29. A medicalassist system, comprising: a primary device housing; said primary devicehousing having a control circuit therein; a lead system to provide anelectrical path between a tissue region and said primary device housing;and a shielding formed around said lead system to shield said leadsystem from electromagnetic interference; said shielding being formed ofa first layer of an apertured conductive material; said aperturedconductive material having a maximum aperture dimension of 0.01millimeters to 10 millimeters, and a second layer of an aperturedconductive material said second layer of apertured conductive materialhaving a maximum aperture dimension of 0.01 millimeters to 10millimeters; wherein the apertured conductive materials of the firstlayer are patterned in a direction substantially parallel to an axis ofthe lead system such that each of the apertured conductive materials isphysically separated from the other conductive materials of the firstlayer; wherein the apertured conductive materials of the second layerare patterned in a direction substantially parallel to an axis of thelead system such that each of the apertured conductive materials isphysically separated from the other conductive materials of the secondlayer; and wherein the apertured conductive materials of the first layerand the apertured conductive materials of the second layer are patternedin a direction substantially perpendicular to the axis of the leadsystem such that each of the apertured conductive materials of the firstlayer is electrically isolated from the apertured conductive materialsin the second layer.
 30. The medical assist system as claimed in claim29, wherein said conductive material is a metal to shield said leadsystem from electromagnetic interference.
 31. The medical assist systemas claimed in claim 29, wherein said conductive material includesmetal-coated carbon filaments to shield said lead system fromelectromagnetic interference.
 32. The medical assist system as claimedin claim 29, wherein said conductive material is a polymer composite toshield said lead system from electromagnetic interference.
 33. Themedical assist system as claimed in claim 29, further comprising a non-permeable diffusion resistant biocompatible material covering saidshielding.
 34. The medical assist system as claimed in claim 29, whereinsaid shielding formed around said primary housing to shield said primaryhousing from electromagnetic interference.
 35. The medical assist systemas claimed in claim 34, wherein said conductive material is a metal toshield said lead system and said primary housing from electromagneticinterference.
 36. The medical assist system as claimed in claim 34,wherein said conductive material includes metal-coated carbon filamentsto shield said lead system and said primary housing from electromagneticinterference.
 37. The medical assist system as claimed in claim 34,wherein said conductive material is a polymer composite to shield saidlead system and said primary housing from electromagnetic interference.38. The medical assist system as claimed in claim 29, wherein aperturesof said apertured conductive material of shielding are constructed fromnon-conductive materials being dispersed in said conductive material.39. The medical assist system as claimed in claim 38, wherein saidnon-conductive materials are randomly located.
 40. The medical assistsystem as claimed in claim 38, wherein said non-conductive materials arelocated in a predetermined segmented pattern.
 41. The medical assistsystem as claimed in claim 29, wherein said first and second layers areconstructed of first and second patterned sheaths, each patterned sheathhaving non-conductive materials and conductive material.
 42. The medicalassist system as claimed in claim 41, wherein apertures of saidapertured conductive material of shielding are constructed fromnon-conductive materials being dispersed in said conductive material.43. The medical assist device of claim 29, wherein the first layer isimmediately adjacent to the second layer.
 44. A medical assist device,comprising: a primary device housing; and said primary device housinghaving a control circuit therein; a shielding formed around said primarydevice housing to shield said primary device housing and any circuitstherein from electromagnetic interference; said shielding being formedof a first layer of a an apertured conductive material; said aperturedconductive material having a maximum aperture dimension of 0.01millimeters to 10 millimeters, and a second layer of a aperturedconductive material said second layer of apertured conductive materialhaving a maximum aperture dimension of 0.01 millimeters to 10millimeters; wherein the apertured conductive materials of the firstlayer are patterned in a direction substantially parallel to an axis ofthe lead system such that each of the apertured conductive materials isphysically separated from the other conductive materials of the firstlayer; wherein the apertured conductive materials of the second layerare patterned in a direction substantially parallel to an axis of thelead system such that each of the apertured conductive materials isphysically separated from the other conductive materials of the secondlayer; and wherein the apertured conductive materials of the first layerand the apertured conductive materials of the second layer are patternedin a direction substantially perpendicular to the axis of the leadsystem such that each of the apertured conductive materials of the firstlayer is electrically isolated from the apertured conductive materialsin the second layer.
 45. The medical assist device as claimed in claim44, wherein said conductive material is a metal to shield said primaryhousing from electromagnetic interference.
 46. The medical assist deviceas claimed in claim 44, wherein said conductive material includesmetal-coated carbon filaments to shield said primary housing fromelectromagnetic interference.
 47. The medical assist device as claimedin claim 44, wherein said conductive material is a polymer composite toshield said primary housing from electromagnetic interference.
 48. Themedical assist device as claimed in claim 44, further comprising anon-permeable diffusion resistant biocompatible material covering saidshielding.
 49. The medical assist system as claimed in claim 44, whereinsaid conductive material comprises nanotubes to shield said lead systemfrom electromagnetic interference.
 50. The medical assist device asclaimed in claim 44, wherein said conductive material comprisesnanotubes to shield said primary housing from electromagneticinterference.
 51. The medical assist system as claimed in claim 44,wherein apertures of said apertured conductive material of shielding areconstructed from non-conductive materials being dispersed in saidconductive material.
 52. The medical assist system as claimed in claim51, wherein said non-conductive materials are randomly located.
 53. Themedical assist system as claimed in claim 51, wherein saidnon-conductive materials are located in a predetermined segmentedpattern.
 54. The medical assist device of claim 44, wherein the firstlayer is immediately adjacent to the second layer.